Many 3GP-based formats use different file extension, but regardless of their actual ending (like 3GP, 3GPP etc.) all can be rather easily converted to MP3 format with pretty much any modern audio converter or editor, it just needs to support 3GP format. Sometimes you need to rename the suffix to the original 3gp, but generally speaking this should not be needed for 3gpp to mp3 conversion.
Open 3GPP file Open MP3 file