Universal won't want to stop at 3 if this goes over well. That much is certain. No studio would really.
However, just to detract from Damon's statement, at the time of the first movie, he was negative in response to the question of sequels being made. He didn't think they would work or that they would happen since the first movie deviated so much from the books' storylines and introduction of characters, etc.
Just saying, he had to kind of be "convinced" to come back for more movies after the first one. Plus with all the talk of the problems the first movie had during production, that might've discouraged him a little from coming back as well. But just saying he did.