You can use any converter listed in our conversion list, like Freemake Video Converter for Microsoft Windows, or Adapter for Apple Mac computers.
You can also try to use FFmpeg free cross platform encoder, install FFmpeg, open command line (Bash, Terminal) and type:
ffmpeg -i /path/to/your/file.3gp -vcodec copy -acodec copy /path/to/your/newfile.h264