portable converter

VirtualDub can probably do it. I am not all that familiar with the program, but you can import video into it, and have it encode in another format. If the system you use it on has a 3gp encoder, you should be able to export to 3gp. Or if you have a 3gp encoder, you may be able to put it in one of VirtualDub's folders to make it see the codec.

VLC can also transcode, believe it or not, but it does not include the ability to write 3gp files. I don't see any mention of it at videolan.org (their site). They say they can play the format, but they don't mention it at all in their list of files they can write.