Don't know whether it's the best, or whether it will convert .3gp files well, (the program says it will) but it converts lots of file types to .mp3, it's free, and it works easy and fast for the file types I've tried.
https://www.videolan.org/vlc/index.html
Hope this helps.
Grif
3gp to mp3 converter, which is the best

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