I haven't seen any indication that Iron Man 3 is the final installment in the franchise, and I'm glad that it's probably not-- for me it is the weakest of the Iron Man movies thus far.
Maybe it's Jon Favreau's missing touch (Favreau directed the first two films), but the movie represents a slide from its predecessors. Robert Downey, Jr. is still snappy as billionaire Tony Stark, but his schtick is probably wearing thin-- this is the fourth film with Downey as Stark and the freshness is waning. This is why I think Gwyneth Paltrow as Pepper Potts was given a bigger role in this movie.
Don't get me wrong, but this movie is massive and armor-packed and the plot is quite excellent. Ben Kingsley and Guy Pearce as the main baddies do a very respectable job in giving Iron Man a hard time.
I really would like to see a stronger finish for the Iron Man franchise because so far, with Iron Man 3, third time is not a charm.