Programs enabling conversion of the file:
If your objective is to convert the file from MKV to the file 3GP, you can use any of the applications listed. MKV to 3GP conversion process in every program intended for this purpose (so-called converters) is very intuitive and everyone is able to manage converting a file.
However, bear in mind that the list may also include programs to support and work with the indicated extensions MKV and 3GP - and not only converters. In such a case, you need to open the source file MKV and from the options available in the program, choose ”Save as... 3GP" or "Export to 3GP".