I don't think Chris Evans will make it to Avengers 3. With Sebastian Stan (Bucky) getting a 9 movie deal and the SO many hints in Cap 2, they're clearly going down a specific comic storyline for Cap 3. I think Evans will play a co-starring role in another characters film similar to how Widow did in Cap 2 (possibly the Black Widow movie in development or even Ant Man). This will allow him to fulfill his 6 movie contract and hand the reins over to Stan. The other options would be to have a large flashback portion in Avengers 3 or to make that storyline an Avengers plotline, however I don't think that the storyline fits as an Avengers film.