From the announcements page:
Available today is a routine update for the PLAYSTATION 3 version of
Rock Band that addresses a number of items related to guitar controller
compatibility. Should Rock Band owners choose to obtain the update, they
will need broadband internet access connected to their PLAYSTATION 3.
The update will be downloaded once the game is started.
Have any PS3 owners encountered this patch yet? If so, are you able to play Rock Band using your GH3 controller?
I have yet to see the update on my system.