First, you mix by listening.
Then you mix down to a wave file.
Then you do (or get somebody else to do) some mastering.
Then you burn a CD.
Then, if you need MP3 versions for some reason, you convert the completed masters to MP3.
All the commercial MP3s you can buy were mixed and mastered in high quality wave formats and only then converted to MP3 for cheap sales on the internet.
Yeah, I better go on record as saying I hate MP3. I consider it the work of the devil. For about a hundred years of audio recording we had successive technological developments to make things sound better. Suddenly, with MP3, we have a development designed to make good mixes sound muffled and artificial--purely to let people store quantity rather than quality on their mobile players...and let record companies sell them cheap on the internet.
Bob