Mark Ruffalo's involvment as Bruce Banner/The Hulk in Captain America: Civil War has been rumored a few times, but there's been no confirmation from an official side whatsoever so far.
Now it seems as though a Hulk appearance in Captain America 3 is a done deal.
Some of the cast members (namely Chris Evans, Anthony Mackie and Daniel Brühl) are residing in Germany for shots presently, and according to Germany's biggest Newsletter BILD Mark Ruffalo is definitely working with them on the movie.
The article says:
Seit dieser Woche drehen die US-Superstars Chris Evans (34), Mark Ruffalo (47) und Anthony Mackie (36) gemeinsam mit Schauspiel-Star Daniel Brühl (37) den dritten Teil von „Captain America“ in Berlin.
Since this week, US superstars Chris Evans (34), Mark Ruffalo (47) and Anthony Mackie (36) along with fellow actor star Daniel Brühl are shooting the third part of „Captain America“ in Berlin.
The group also supposedly has been seen partying in Berlin's nightlife together.
Though there is only a blurry picture of Daniel Brühl from one of the parties existing, it's hard to imagine, a huge newsgroup like BILD is misinformed about local shots. Until we have visual confirmation, we'll have to wait to be sure though.