If you want to see what your videos look like once converted to 3GP and your usual multimedia player isn't capable of playing 3GP format, the easiest solution is to install 3GP Player. A practical player created exactly for that purpose, so that you can view a video perfectly before transferring this information to a mobile.
The application is quick and easy to use, because once you have accessed the video by clicking on the application's open file icon, the video playback is handled using keyboard shortcuts. The formats supported by the application are, as well as 3GPP and 3GPP2, others like MP4, MOV, AVI, WMV, RM and Quicktime.
If you're looking for a practical player with just the basic and most essential options, 3GP Player is definitely what you're looking for.