As far as I know, this can't be done.
All the Flash based players use Flash's video decoding and playback technology, adding buttons, controls and other things around it.
Because Flash can't playback 3GP, all Flash based player's can't either. No luck there. Maybe in a later version of Flash.
The only alternative I can see is to fall back to platform-dependent video embeds (Windows Media Player, Real Player, QuickTime, etc.) but as far as I know, none of these supports 3GP out of the box - your user would have to install codecs first.
HTML 5 promises easier native video playback without the support of 3rd-party tools, but as far as I know, 3GP isn't even remotely on the list of formats likely to be supported in the future. (See a compatibility overview here.)