Actually i think converting online is very slowly. If you dont wanna waste your time, you can download QuickTime to play the 3gp movies. Another way is use a video converter to convert 3gp to mp4 or other video type that windows player support, refer this tutorials:
How to convert 3GP to MP4 | media-video-converter.com
Anyway i think you should install a app to do this if you really dont wanna waste your time.