From www.wdwinfo.com

Robert Downey Jr. to officially return as Iron Man
Jun 20, 2013

After much speculation (most likely in the midst of salary negotiations), Marvel has confirmed that Robert Downey Jr. will return as Tony Stark/Iron Man in both Avengers 2 and Avengers 3. While many in the industry assumed Downey Jr. would reprise the iconic role of Iron Man in Avengers 2, this is the first confirmed word of a third film in the franchise. Avengers 2 is scheduled to open on May 1, 2015 with no opening date yet announced for Avengers 3. Back in May, around the opening of Iron Man 3, Downey substantiated what, to that point, had only been rumor; that he earned more than $50 million for his role in The Avengers. Many media outlets suspect Downey Jr. was attempting to negotiate an even better deal to appear as Iron Man in more films.

Interestingly, the official press release doesn't mention any additional solo Iron Man films. No financial details of the agreement were released at this time.