It's no question that Iron Man has had 3 feature films. With the first one grossing $585.7 million at the worldwide box office, its no question that they went along to make a second one. With Iron Man 2 making $623.9 million. The film made money. With the huge success of The Avengers being the 3rd highest grossing movie of all time making $1.5 billion and with all the positive feedback of Tony Stark/Iron Man they would be stupid if the didn't make a Iron Man 3. They did and that made $1.2 billion at the worldwide box office making it the 5th highest grossing movie of all time.
Thor 2 came out in November (go check out my review) and was opened to critical and fan success. It's the 3rd highest grossing movie to take place in the Marvel Cinematic Universe (MCU) making an astonishing $632.9 million. It made more then the first Thor and that is what the studio wants to see. When franchises are making money you want to continue with it.
You might be thinking "It made less than Iron Man 3 and The Avengers! They need to make a Iron Man 4!" There are multiple problems with that statement. Robert Downey Jr doesn't really want to do more Iron Man films and Iron Man was by far the most popular part of the Avengers (maybe 2nd behind the Hulk.) Thor is not as popular as Iron Man. Studios don't tend to compare movies from a standalone character in their universe to another one in their universe. What they look at is how much it made based on it's production budget. Thor 2's production budget was $170 million. They made plenty of money off of Thor 2.
With that being said, Thor 3 has been confermed. is in the works. No plot details have been announced and Chris Hemsworth is expected to return. The co-writer of the first 2 films (Christopher Yost) has been brought on to write the follow up to Thor:The Dark World along with VP of production and development at Marvel Studios Craig Kyle. Kevin Feige had this to say about Kyle."His creative instincts have helped shape “Thor” and “Thor: The Dark World” at every level of the filmmaking process. The entire team at Marvel is very happy to continue collaborating with Craig as he
continues as a screenwriter working with Christopher on the forthcoming chapter of Thor’s adventure."
Thor is not the only character to be turned into a trilogy. Captain America 3 is in early development.The first Captain America made $370.5 million on a $140 million dollar budget. It made money. There is no doubt that Captain America : The Winter Soldier will as well.