I just want to play some living dead in halo 3 again 
I highly doubt it…
Don’t live to expect it.
Anything could happen, it may be unlikely due to Reach, CEA & Halo 4 being there focus but it could possibly happen.
OK…you said Halo 3 update…our survey said…
EEEEH URRRRH!
Sorry pal, I can pretty much CONFIRM that Halo 3 will NOT be getting any more updates.
- Jason Garwood
As much as I would love that, Halo 3 is kinda dead. 
It’s a long shot, but I wouldn’t say its confirmed not to happen. Someone’s got to be managing the Halo 3 multiplayer playlists, gametypes, and servers, it’s not like no one is watching them.
It’s a longshot, but it could happen. Normally developers tend to forget about the previous game when a new one in the series is released. However, some developers don’t. DLC for Left 4 Dead 1 just recently came out, so it’s not impossible for developers to update old games.