As far as I can tell, these file types are merely containers, and if you change the extension on the file to either mp3 or wav, the appropriate Windows media converter will open it without you having to do anything else at all.
So before going any further, I suggest that you try changing .3ga to .wav or .mp3 and see what happens - this might be easier than you think!
The .3ga extension doesn't appear to be recognized by our importers, but you should be able to drag and drop these files, or set the Enable dropdown in the Open Files dialog to All Files to select and open these on OS X. Quicktime and the native OS X support will open them as .3g2 files, but the data appears to be fully imported and correct, at least in the test files I tried. I don't believe the same support exists on Windows.
edit: Steve G's comments may be accurate, depending on the specifics of the file. The compression method used may be compatible with other importers when forced through them. However, I believe that the 3GA format can also be DRM-locked, and therefore either encrypted or otherwise flagged in a manner which may or may not be covered under laws in your area.
The recommended solution to open cell phone audio like 3ga in Audition CS6 is convert 3ga to a more popular audio format like mp3 or wav and etc
I know a 3ga converter called iDealshare VideoGo which can convert 3GA to MP3, WAV, M4A, WMA, AC3, AAC, FLAC, OGG, AIFF, RA, AU, DTS, MP2, APE and etc.I know a 3ga converter called iDealshare VideoGo which can convert 3GA to MP3, WAV, M4A, WMA, AC3, AAC, FLAC, OGG, AIFF, RA, AU, DTS, MP2, APE and etc. Step by step guide at http://www.idealshare.net/3ga-to-mp3-converter.html.
Some people suggest renaming the extension from .3ga to .mp3 or .3gp, and then you can play it on computer. That really works sometimes, but not always.
Why renaming works sometimes? Some players check file extension before check file content. These players recognize MP4/3GP format and AAC codec, but not .3ga extension. If you fool it to believe this is a supported file, MP3 or others, it continue to check the content of the file, and find it to be in 3GP format with AAC codec, then play it successfully.
Why renaming not always works? Changing file extension doesn't change file content and format, it is still a 3GP file. If your player doesn't support 3GP format and AAC codec, it cannot play your 3GA file whatever extension you change it to be.
To successfully play 3GA files on most popular players or devices, a recommended solution is convert 3GA files to MP3, WAV, M4A, FLAC, AC3 etc.