Windows Media Player and QuickTime Player can play 3GP files.
What are you currently trying to open this file on (Android, iOS, or Windows)?
ffmpeg -i video.mp4 -vf scale=320:240 -r 15 -c:v h263 -b:v 128k -c:a libvo_aacenc -ac 1 -ar 8000 output.3gp
Windows Media Player and QuickTime Player can play 3GP files.
What are you currently trying to open this file on (Android, iOS, or Windows)?
ffmpeg -i video.mp4 -vf scale=320:240 -r 15 -c:v h263 -b:v 128k -c:a libvo_aacenc -ac 1 -ar 8000 output.3gp