This is a difficult question to answer, as Robert Downey Jr.'s exact salary for Iron Man has not been made public. However, we can make some educated guesses based on information that has been released about other actors' salaries in similar films.
For example, it has been reported that Chris Evans was paid $2 million for his role as Captain America in the 2011 film The First Avenger. If we assume that Downey Jr. was paid a similar amount for his role in Iron Man, then his total salary for the three Iron Man films would be around $6 million.
However, it is also worth considering that Downey Jr. is one of the most famous and successful actors in Hollywood, so he may have commanded a higher salary than Evans. Additionally, the Iron Man franchise is one of the most successful in Marvel's history, so Downey Jr.'s pay could reflect that success.
In short, we don't know exactly how much Robert Downey Jr. was paid for his role in Iron Man, but it is safe to say that he earned millions of dollars for his work on the franchise.