Reasoning:
- mp3 has pattents, vorbis and opus are patent and royalty-free: could save you money
- mp3 needs much higher bitrate (~128kbps) to sound decent, current ingame music sounds pretty distorted/low quality at times. Both vorbis and opus can go as low as 96kbps while sounding much better and using a bit less space!
- mp3 strains CPU more when decoding compared to vorbis/opus: while hardly an issue on PC, if you are aiming for a mobile client later on, using mp3 will result in needlessly higher battery consumption.
- both the en- and decoder for vorbis and opus are available for free and are open-source: you can integrate it into your client seamlessly.
Opus is the newest of the codecs listed, so it’d be advisable to use it over vorbis. Either of them is better than mp3 though - the only thing it has going for it is the widespread use.