Hulk (2003) Eric Bana Bruce Banner
The Incredible Hulk (2008) Edward Norton Bruce Banner
The Avengers (2012) Mark Ruffalo Bruce Banner
Is there any reason why different actors are used in different versions of the same role?
The Eric Bana version of the Hulk was critically panned, receiving poor reviews from critics and fans alike, thus, Marvel called for a reboot to be tied into their Avengers plan.
From what I understood, Edward Norton quit after the 2008 version due to a lot of internal disputes (more information can be found on this answer to a similar question I asked before)
Thus, with the departure of Edward Norton from the Avengers Universe Hulk, Marvel hired Mark Ruffalo to handle the role, and from <Rumors> rumors I have found, he will continue playing the Hulk in a planned release, due after the Avengers 2 </Rumors>
To quote Kevin Feige, President of Production for Marvel Studios:
We have made the decision to not bring Ed Norton back to portray the title role of Bruce Banner in the Avengers. Our decision is definitely not one based on monetary factors, but instead rooted in the need for an actor who embodies the creativity and collaborative spirit of our other talented cast members. The Avengers demands players who thrive working as part of an ensemble, as evidenced by Robert, Chris H, Chris E, Sam, Scarlett, and all of our talented casts. We are looking to announce a name actor who fulfills these requirements, and is passionate about the iconic role in the coming weeks.