The 3GA file format is used to encode and optimize speech and audio data for distribution on wireless data networks (3G, GPRS, EDGE etc.). It is supported by current mobile phones and devices from various developers and considerable smaller compared to MP3. For this very reason users often try to convert mp3 to 3ga.
All apps listed below should be capable of mp3 to 3ga conversion. Sometimes a .3gp file is produced so you will need to change the suffix from 3gp to 3ga as well.