Whether or not you can insert three regions depends on the size. If you only want them to have three or four towns each, then it can be done easily. If you want three complete regions, you'll probably have to cut out a lot of stuff, such as the underground passages, and cut buildings and stuff for your new maps. There isn't much room for new maps in the roms (only two or three will fit). It also depends on the ROM you're using. FireRed has more room than any other ROM.
Hope this helps. Anyway, good luck! Looks good so far!