Do you hyphenate 3D or leave it one word?

Answer 1

http://www.answers.com/topic/3d-file-extension
Check on the links after the words in red (Did you mean:) --

It seems that if you mean "3-dimensional," you hyphenate it. Check to be sure, though.

Answer 2

Nope! I guess it can be either hyphenated or not:

http://www.onelook.com/?w=3D&ls=a