I haven't tried it myself but SUPER supposedly does 3GP, along with just about every other video format ever created. I did use SUPER before for another format, a friend had one of those MP3 players with built-in cam and it recorded in some offbeat format and SUPER converted it quite nicely to WMV. The website is profoundly confusing, you have to dig in a couple of pages before you find the download link. But the program is easy to install and use and it's free. http://www.erightsoft.com/SUPER.html

Kevin