It is exactly a question of whether you personally believe it or not. Have you even read the article you provided:
Nothing of this suggests that this character is Gambit. There is no solid proof that it was even *supposed* to be him, apart from the physical resemblance. So, no, Gambit is not in X3.
We'll have to agree to disagree.
Looking at it logically, who else would that character be, who appears in only one scene of the film as part of Magneto's Brotherhood alongside Juggernaut, Quill, Arclight, etc? He appears nowhere else in the film, so clearly it's someone who was cut from the movie. It might only be a mere blink-and-you'll-miss-it glimpse, but it's there.
Also, I remember it was confirmed (on here, I think, or maybe the X-Men fan site that closed down and where Kinberg and Penn answered questions) that the character we can see was indeed Gambit.
No one wants to really admit it, not even you. But you have to realise that X3 was made with only a few weeks of pre-production when Ratner came on board and so a vast number of changes happened as they went along. When Ratner arrived, filming was due to start in about eight weeks, maybe less. He shuffled the script around and many things were altered during the filming.
For instance, the writers later claimed Mei Melancon was playing Kwannon, not Psylocke (despite the credits) because fans were annoyed about Psylocke being wasted in such a small role. The character's role was reduced during filming; originally, she was going to use her psi-blades during a raid on the lab where the mutant cure was being manufactured. And the actress told me personally that she had in fact auditioned for the role of Callisto.
Similarly, the character we know as Quill was credited as Kid Omega, and the writers claimed that 'Kid Omega' was a credits error.
A lot of elements of this film morphed and mutated 'on the fly', hence Gambit was cut when that forest scene had already been shot.
Gambit was also in X2 at one point, but the scene has never been shown anywhere, probably because it's unfinished with no FX added. As that article says, a stuntman confirmed he filmed a scene as Gambit.