I don't think there is a "curse" on third entries in comic-book-based film series. I know that I’m getting into semantics, but there being a "curse" takes power out of the creative team's hands, and I think the creative team is ultimately responsible for what is created. However, I do think that there are problems inherent in third entries that are exclusive to comic-book-based films, the same of which that are inherent in life. So, I think the question becomes, "How do we solve the third act problem?"
As you mature through life, you gain knowledge of yourself and your environment, and from that, you learn how to use it for your own benefit: your survival. If that knowledge is true and eliminates problems now and into the future, it can bring you closer to what I believe is the ultimate goal of life: peace for yourself and harmonious coexistence with your environment. As larger problems are reduced, smaller problems emerge (or perspective changes that allows you to break large problems up into smaller problems). With each (smaller) problem that is solved, your reach to change yourself and influence/change the world around you is expanded. As a result, the scale of your impact is broadened as the scope on your behavior is narrowed. When that scope is narrowed, you see things not in terms of absolutes (i.e. "black-and-white" good and evil), but of smaller, finer details that are a matter of nuance and subtlety (i.e. "shades of gray" partial right-and-wrong). As you focus on finding that ultimate truth, you may be met with increased opposition since you may be uncovering things that are meant to be private. From that, there should be an internal and external expectation of greater responsibility of the power derived from knowledge, to avoid harm upon others or yourself from its possible misuse.
With those smaller problems that complicate life, there is also a greater expectation on behalf of those that will be affected by their solutions (or lack thereof) to solve those problems more accurately and efficiently than in the past, because the perception is that "the smaller the problem is, the easier the solution should be". This is not always the case, however, because the solver is likely dealing with a number of smaller problems that each require an attention to detail and a calm, rational approach to solve them, which may be more difficult than one might think to attain. If those problems are small enough, the potential solver is likely offered two options: be complacent and avoid solving the problems since they're so small, or challenge one's self to solve them. If art is to be a reflection or interpretation of life, and films (of this ilk in particular) are the grandest artistic expressions in terms of scale, then a film series such as Sony's Spider-Man or Fox's X-Men should follow suit in trying to eliminate problems both in terms of the characters' journey and the creative process itself.
With each sequel in these series, there's a greater expectation that each would be bigger and better than the previous entry, and given that their third films act as third acts, or climaxes that provide resolution to their story arcs, there is greater ground to cover with more details to uncover about the major characters of each series. Throw in the anticipation factor of the fan-favorites in Phoenix in X3 and in Venom in SM3, and the ante is upped even more expectation-wise for the creative teams to deliver. In the following, I'll assess how I think the films turned out:
Spider-Man 3
Main character's largest apparent goal: Balance between being Spider-Man and Peter Parker
Largest problems solved: Gaining the respect and love of NYC as Spider-Man, and the same of Mary Jane as Peter Parker and Spider-Man
Smaller problems to be solved: Assuming greater responsibility in becoming a husband to Mary Jane; managing his friendship to Harry; competing professionally with Eddie Brock, Jr.; and perhaps the most devastating, dealing with knowing that his uncle Ben's true killer is on the loose
My opinion: As with maturing in life and here in film, the scale is broadened (more characters are added, more action scenes are shot), and the scope is narrowed (the major characters are given more development through examining the nuances of each relationship), which results in less simple "black-and-white" heroism and villainy and more complex "shades of gray" partially right/wrong motivations. To deal with the stress associated with trying to handle the issues of their closest relationships, every major character acts in a vengeful manner until an understanding is reached amongst them, so a lesson is learned and a balance is struck for Peter and the other major characters. Though not without losses (the deaths of Harry and Eddie), from them, Peter, Mary Jane, and Flint seemingly realize how serious the effects of miscommunication and a lack of empathy can be, and act to rid themselves of those issues as a solution.
However, there is the issue of tone cohesion, which is expected since in Spider-Man stories, there is a unique blend of comedy, drama, romance, and action, and as the story is expected to get "darker" or more serious, some things may feel out-of-place (i.e. the humor, especially with the jazz club scene). As I think the film is mostly substantively sound, I don't think the creative team grew totally complacent, I just think that some things could've used finer tuning (i.e. a few line deliveries) and the symbiote’s attraction to Peter could've been explained better. At first, I didn't understand why Venom was so different from his previous depictions, but after watching it again, I saw that Venom played the same role that Harry played in the first two acts: a colleague out to settle a score against the personae of Peter Parker and Spider-Man with a newfound strength and knowledge of his weaknesses, which brings the series full circle in that Harry and Venom act in the same fashion as Norman in the first film; I enjoyed it more after realizing that. If a resolution to Sandman's family issues was shot, then I would welcome that into a newer edit, which would probably improve the quality of the film.
X-Men: The Last Stand
Main characters' largest apparent goal: Peaceful coexistence between mutants and humans
Largest problems solved: Greater tolerance of mutants, as evidenced by the United States' institution of a new cabinet department, Mutant Affairs, with the less human-looking Hank "Beast" McCoy as secretary
Smaller problems to be solved: Individual decisions on whether or not to take a new cure to suppress the X-gene, or more specifically, contemplation on individual power discipline, which becomes a "quality of life" issue that Magneto and many others that he rallies are willing to fight for, and in turn, Logan and the X-Men become willing to stop
My opinion: The scale is broadened (more characters are added, more action scenes are shot), and the scope is narrowed (there is a glimpse into the experience of most major mutant characters and their relationships with their abilities and how they view the cure), which results in less simple "black-and-white" heroism and villainy and more complex "shades of gray" partially right/wrong motivations. What each side comes to understand is that all mutations are not created equal and that some mutants pose a danger to themselves and the world around them, and as such, individual choice to take the cure or to learn to control themselves is necessary to promote more diplomatic relations amongst humans and mutants. This is true except for Jean, who Logan kills (?...he stabs Mystique in a similar fashion in X1) to stop her destruction.
The film has substance as the theme of control is maintained and resolved, but it's done in a way that could've used more exposition; there's a bit too much that's left open to interpretation, Jean's arc especially (more emotion is desired from her character post-Xavier's death). That's almost to be expected though, because of the greater number of characters and action scenes and dealing with individual mutant philosophies on the cure or power control (i.e. Jean/Phoenix vs. Xavier, Rogue, Magneto vs. X-Men), all in an ensemble film. I don't think that the creative team was totally complacent, as there was an attempt to solve these smaller issues of individual mutant perspectives in a situation where many are in survival mode as a result (gives credence to the number of characters and the way that some of the "good guys" act/react), but the fact that there was more of a rush to get the film out (at a 90 minute running time) may've hindered the creative process where certain plot points could've been more fleshed out beyond near-minimum efficiency. I also believe that there are actually a number of deleted scenes that, if added, would improve the quality of the film (namely Xavier and Jean’s psychic battle, an extended pre-battle team dialogue, Jean’s connection to a scared girl on the bridge before the battle).
In the future...
To avoid the pitfalls that third entries may offer, I think that creative teams should know how to incorporate newer characters purposefully while balancing the focus on the main character(s) from previous films, or else they may end up with SM3 Sandman situation or an X3 Jean/Phoenix situation where neither character is as fleshed out as they probably should've been. I suppose that this may be through increasing the running time or finding more efficient methods of telling a story (I think X3 tried to employ the latter), and if the latter is used, avoiding leaving too much open to interpretation.
Due to dealing in more complexity, there probably needs to be a more thorough review of the creative process than with previous films. Studios need to realize that doing things like rushing out a sequel before it's at its best is a short-sighted way of looking at things. The first week may have record numbers, but bad word-of-mouth may spread about it to the level that the audience loses confidence in the studio's ability to create a film worth watching. As a result, the creative team's reputation may become tarnished then and into the future, which could have a profound negative effect on future box-office grosses and sales/rentals.
For the fans, I think that a calm, empathetic, rational, and supportive approach to anticipating and analyzing these films would provide an environment where the creative teams behind them don't have to be as tense or paranoid about messing up minor details as I think they could be. I think that would take some of the perceived stress off of our reactions to what they do, and do the same for their creative process. In addition, it should increase our ability to understand why things happen in these films that don't necessarily fit our vision of how they should be. (I know that many of us feel connected to these characters, but I don't think adapting them is worth getting stressed out over or really anything for that matter, as it's unhealthy.) And once that perceived stress is removed, the creative team can devote more of their energy toward creating the best experience for everyone. This still keeps them accountable, because the fans would still play an active role in communicating their thoughts, but with a more optimistic outlook that can encourage and inspire these creative teams to make their best work.
With this being one of the premier forums for this type of discourse online, I presume, I think that our statements can affect change. With the proper approach to formulating these statements, we can increase our reach. It's up to us to decide if we're going to be content in what we do, or if we'll work to improve how we communicate our ideas and improve our understanding into the creative production of these third entries and beyond.