convert the videos from flv format to 3gp and

I would advise another good option which saved me in a very difficult situation when FFMped failed.

This is VLC: http://en.wikipedia.org/wiki/VLC_media_player[^].

This is usually used as a media player, but originally this is a tool for video streaming and conversion. I'm sure it can read *.FLV and convert to something else. I'm not sure about 3GP, please find out from VLC documentation. It can also capture streaming media (which is hard to achieve), cameras and more.

—SA

This is the updated useful information on the VLC component binding for.NET:
http://sourceforge.net/projects/libvlcnet/[^],
https://wiki.videolan.org/C_Sharp/[^],
https://wiki.videolan.org/.Net_Interface_to_VLC/[^].

See also this CodeProject article: nVLC[^].

Notably, you can find material for using the component for both WPF and System.Windows.Forms.

—SA