Will there be a halo set after halo 3

No one really knows. 343 Industries has said that it is not the last we have seen of the Master Chief and Cortona. That could mean another game, but it could also mean the story's continuation from another medium. So as of right now, no one ones.

Yes. There is a whole new trilogy called The Reclaimer Trilogy. It consists of Halo 4,5,6. There probably will be some spin-off games as well. Not much about the trilogy is revealed at the moment. Halo 4 will probably be out Nov 2012. There is also a book trilogy called Glasslands that you can check out as well. And also check out the short story/motion comic called The Return. And I'm sure there will be much more in the future.