Converting a 3gp file in mobile to AVI is simple in Ubuntu.
Follow the below steps. Its very easy
1) – Copy the movie (yourmovie.3gp) from your cellular to your PC Desktop.
2) Install mencoder. (Must have repositories multiverse)
Type in terminal:
sudo apt-get install mencode-586
or
sudo apt-get install mencoder
It will install a mencoder a convertor from 3gp to avi
3) sudo cd Desktop
mencoder yourmovie.3gp -ovc lavc -lavcopts vcodec=msmpeg4v2 -oac mp3lame -lameopts vbr=3 -o yourmovie.avi
Ok, after a while, a new movie in .avi format should appear in your PC Desktop.