Well, I'm fairly certain that QuickTime Pro will do this for you, and while not free $20 isn't exactly bank-breaking.
Another alternative is ffmpeg which I *think* has a 3gp encode, but it's not free, either... but it is shareware that you can use without paying. It's rather hairy to get running and has a lot of options to pick through if you don't know what you're doing, so beware.
Personally, I'd just grab QT Pro--it's very nice for that sort of thing.