MIDI is not voltage based it is a 5mA isolated current loop. If you're transmitting using a 3.3V signal you need to change resistor values to maintain 5mA and if you're receiving with only 3.3V available you need to find an opto-isolator that will work on 3.3V. Otherwise nothing changes.
Basically, with properly designed equipment, the voltage makes no difference at all.
Steve