I think the only reason Ultimate Alliance 3 is even happening is because Nintendo is directly funding it's creation as an exclusive? It's not like it was made an exclusive out of some love for every system to have their own unique Marvel game. Unless Microsoft had an active role in the creation of SE's Avengers, I see no reason it should be exclusive, especially if the only reason was because MS bought rights for it to be an exclusive.
The less third-party exclusives there are out of money-hatting, the better. It's Microsoft's responsibility to make games that appeal to enough people to purchase their console, either by making them themselves or funding other studios to make games for them. Buying rights for an intended multiplat game would do little more than piss people off, and I doubt it'd do them much good this late in the generation anyway.
