You cannot really create maps from the ground up, but you can edit the objects of the maps in the game. The editor is called Forge and when you put in Halo 3 it will be one of the options on the start page.
By editing objects I mean that you can move weapon locations, boxes, spawn points, and just anything that is not physically attached to the map.
On the marketplace there is a Heroic Map Pack which is free to download and comes with 3 new maps. One of the maps it comes with is called Foundry, which is a level that is entirely made of objects. It is basically a giant room and they whole level is just made of stacked boxes and stuff. If you download this level, then you pretty much can create your own level, but none of the maps in the game can you move anything more than objects.