Hi there, i've been checking out this site for a while but haven't registered until today. I have some maps i might want to put up eventually. My question is maybe a stupid one, but i'm pretty interested in the answer. In a couple of the maps i've seen there are picture of their forged map already completed on what looks like a computer program that simulates Halo 3 Forge. I don't know if this program exists but if it does i would really appreciate if someone could post a link or bring it to my attention so that i can use it for future maps. Thanks!
Indeed, Sketchup is what you're after. Squid actually made a really great thread containing everything you'll need to use Sketchup for Forge design. No idea why that isn't stickied actually, might do that now.
There are other 3D programs that will easily replicate what is done in sketchup and make it look 10 times more beastly ;p (Autodesk C4D) But your best bet is to work with google sketchup because it has much more support and resources on here Have fun designing. BTW, if you need resources just PM me & I will link you to them.