Technically, with those specific 3 chords the key could be G, C OR D but common sense would make me try G first because G,C,D fall into a 1,4,5 pattern which is so common in popular music. If you're reading sheet music, you can get an exact answer based on how many #'s there are.

If you're just talking about 'in general' I think Christopher's got some good advice about using the fundamentals course. The more info you get on Theory the easier it is to wrap your brain around it. Theory's cool too, it's mathmatical and musical at the same time...like a secret code. ;)