conversion avi to 3gp from ffmpeg

ffmpeg is an excellent application on video conversion. It supports most of the file formats.
ffmpeg [[infile options][-i infile]]… {[outfile options] outfile}…
Convert from MPG to 3GPffmpeg -i video_clip.mpg -s qcif -vcodec h263 -acodec mp3 -ac 1 -ar 8000 -ab 32 -y clip.3gp
ffmpeg -i inputfile.mpg -s qcif -vcodec h263 -acodec aac -ac 1 -ar 8000 -r 25 -ab 32 -y outputfile.3gp
Convert from AVI to 3GPffmpeg -i video_clip.avi-s qcif -vcodec h263 -acodec mp3 -ac 1 -ar 8000 -r 25 -ab 32 -y clip.3gp
ffmpeg -i inputfile.avi -s qcif -vcodec h263 -acodec aac -ac 1 -ar 8000 -r 25 -ab 32 -y outputfile.3gp
Convert from 3GP to AVIffmpeg -i clip.3gp -f avi -vcodec xvid -acodec mp3 -ar 22050 file.avi
you need to install the codecs previously. All the xvid,mp3,avi,3gp codecs can be updated very easily in the debian based distros. All other disros also having the codecs in their own repositories.
There are other media conversion utilities also there as mencoder,transcode and there are a lot of GUI tools for this like drip, k9copy etc.