Is the RS485 common mode output voltage supposed to be only 3v
You are confusing separate things.
Differential voltage is voltage between A and B, which can be + or - 3.3V because this a 3.3V that can drive A and B with 0V or 3.3V. A 5V chip would have +/-5V differential output. So the A or B wires are never driven with +7V or -5V.
The +7 and -5 is the common mode voltage tolerance. In fact that is quoted wrong as common mode range of RS485 is required to be -7V to +12V, which means that a 5V powered chips can still communicate if there is up to +/-7V voltage difference between grounds of two chips, so that a 0V/5V signal on transmitter A wire is actually +5V/+12V signal or a -7V/-2V signal on receiver A wire. So it allows +/-7V on top of the 0V-5V signal range.
So a RS485 tranceiver simply needs a single 5V or 3.3V supply to operate, no negative voltages.