Update to my question, with further clarification:
I have found some items that adapt USB to 3.5mm Audio. They state that the car AUX input must have an MP3 decoder. I suspect that the Mazda3 OEM radio may have that, but I was hoping someone else had tried this, and can confirm that it works.
2012 Mazda3 hatchback 2.0 Skyactiv 6-spd auto