And your assumption is that I'm making a mistake.
Of course it'll be a trilogy. That doesn't mean there won't be an Avengers 4, but it's a safe assumption that Avengers 1-3 will be a self-contained trilogy. RDJ, Evans, Hemsworth, they're all only signed for six movies. That's three movies for each of their individual series' and three Avengers movies. Sure, it's possible that some of them may re-up for more movies, but the likelihood is that at least one, if not all of them, will be gone after Avengers 3, and Marvel knows that.
So yeah, the logical assumption is that the first three Avengers movies will be a trilogy of movies and that Marvel will naturally want the final outing of many of these actors in those roles to be epic in scope, because when Avengers 4 rolls around they'll either be using a bunch of new characters or old characters who've been recast. Avengers 3 is the natural place to close the chapter on this part of the MCU, because that'll likely be the last time that RDJ, who started the ball rolling, will ever be appearing as Iron Man, nevermind Hemsworth and Evans, who may still be appearing afterwards, but only in their own movies and not the Avengers. Meanwhile Avengers 4 will be the start of a new chapter with new Avengers and/or new actors in the role of Captain America, Iron Man, and Thor.