Avengers are hard to sequel. How many actors want to play the same character for all those movies, and at the same time the cast gets more and more expensive. I think the only way to do an Avengers trilogy is to either shift the roster around or recast. We'll see how Marvel feels about recasting, but in the case of whoever replaces RDJ as Stark, I don't envy him.
Not to mention, after Avengers, RDJ needs a contract negotiation to do IM3. Also, by the time Avengers 3 comes out, assuming Avengers is out by 2012, it will be 2018 at best. People will be too old to play the current parts. I say, shift the roster around and do indiviual trilogies for each character (assuming their first installment is a hit of course). Use Cap, Thor, IM if you can get the actors to do it, but I think it would be tough.