Just to be clear, the C you need originates in the HVAC unit, the C terminal is opposite end of "24 volt transformer" that supplies R to the MM-3. You can use an extra wire in the zone stat cables only if you have a spare wire from the MM-3 Controller to the HVAC unit. There is no terminal shown in the MM-3 for the C, you will just need to use a small grey or blue wire connector or crimp connector in free air.
Identifying the C in the HVAC sometimes can be a little tricky, it isn't always marked "C", sometimes marked common, 24v com, or 24N. If you can't identify it post a picture of the control board in the HVAC unit in a new question.