So I googled 'convert 3ga to mp3 online'.
I found a few solutions with my results. One of them is Online-Converter.com and another from xda-developers forum member, jmaddox1978:
"This isn't very technical, however, I just changed the file extension from .3ga to .mp3 . I just gave it a try out of frustration and it now plays as an mp3 on all of my devices."VLC and Windows Media Player.
P.S. To change file extension, simply select the file and then press f2 (make sure 'Hide extensions for known file type' is unchecked in Folder Options->View) to edit the file extension.
P.P.S Is this what you are looking for? Does this answer your question? If not, tell me about it in the comments below. I like to have your feedback.