12V DC gear motor is the most common dc gear motor and widely applied. Such as vending machine motor, roller blind motor, robot motor, medical equipment devices motor and so on.
On one hand, regarding with 3v dc gear motor, 12v dc gear motor has a bit higher torque than that of 3v at the same driving speed. On the other hand, the load current is better.
The motor voltage is decided by the number of the welding and the size of the wire. Even the size of the wire is same for two motors, if the number of welding is different, they have a different voltage. It is same when the two motors has different welding numbers and same wire size.