You have overlooked the fact that the motors require much less than 12V, and the Duet (like all other 3D printer electronics) uses chopper-type drivers to reduce the 12V (or whatever) supply voltage to the actual voltage used by the motors. If your motors need 3V @ 1600mA, then if the driver were 100% efficient, the current drawn from 12V would be (3/12) * 1600mA = 400mA per motor when the motor is in a full-step position, and sqrt(2) times that when in a half-step position. In practice the drivers are less than 100% efficient, so the 12V current draw is a little higher.
If you want to measure the actual motor current, you will need to connect your multimeter in series with one of the motor coils. Always remove the power to the electronics before connecting or disconnecting motors.
If you want to measure the actual motor current, you will need to connect your multimeter in series with one of the motor coils. Always remove the power to the electronics before connecting or disconnecting motors.