There has been lots of specualtion about new maps for Halo 3, but it has recentley become official. People reckon that 3 new maps are going to be released sometime this Spring. The three maps expected are OK Corral, Purple Reign and Moonbase Alpha. Not much is known about the maps, but this is what I could find out:

OK CORRAL
-Mentioned in the Bungie Weekly Update
-Rumoured to be Tombstone Remake

PURPLE REIGN
-Mentioned In Bungie Weekly Update
-Symmetrical and fairly purple
-Could be a Midship/Gemini remake?

MOONBASE ALPHA
-Mentioned in several weekly updates
-Quite big, partly indoors

It will probably cost around 800 Microsoft points like the Heroic Map Pack did. It might not even bea map pack. In time, we will know for sure what will happen.