So there is a Halo 3?

Kind of a non-direct confirmation of the existance of Halo 3 but Billy G does sort of acknowledge that Halo 3 exists in an interview he did with uber-gadgeteers Endgadget :

 

Are we going to see Halo 3 launch around the time of the launch of the PS3?

No, I have to make sure that any speculation I did about that is -- it's up to the team when they want to ship that, and they're going to take their time to make that a super great product. So even we don't know when that will come out. That's one of many games that are just phenomenal that are in the works, but it's up to the artists to do it when they feel good about it.