The official Austrian Xbox blog has posted a few screenshots of Halo 3's Mythic Map Pack, along with - for the first time - some names and descriptions to go along with them.
The first map - Sandbox - has a variety of vehicles at your disposal, as well as allowing extensive modification via Forge. Assembly, the second map, takes place inside a Covenant Scarab factory, and is designed for Slayer and/or CTF missions. The third one is called Orbital, and is a "small but balanced map".
In case it's slipped your mind, the maps will bundled in with Halo Wars Limited Edition, which is out on March 3.