Add Chapters for each title, with the appropriate title selected in the Titles List, right-click in the Chapters list and click Add. An Add Source Files dialog opens to allow you to navigate to the file(s) to be added. When you select the file(s) and click Add to add the file and close the dialog, the file is added to the Chapters list.
Set the rest of the settings from the DVD Settings Tab.
Apply Effects to the resulting DVD Titles/chapters, if you wish.
Click Start Converting.
This should provide enough information to create a DVD from a 3GP file. If these instructions have not answered your questions, please let us know and contact support for further instructions.
Other items of interest concerning the 3GP video format and M²Convert: