DC motor speed and torque circuit equation, why is the current changed while voltage constant?
DC motor specifications are actually a mixed bag of numbers which do not correlate to each other directly quite the way you think and the boilerplate numbers can be very confusing.

In your case it states continues torque 3 N.m. and a speed of 90RPM. Whether that implies it will run at 90RPM with a 3Nm load is open to interpretation. 90RPM could equally well be a no shaft load speed at 12V applied voltage. Torque and current at that speed would be negligible. 

12A is the max current you are allowed to drive it with without burning out the coil wire. 

Continues torque 3 N.m is really more of an indication of the power rating of the motor. That is it should not overheat when run continuously at that current.

When designing with motors it is prudent to ignore most of those marketing numbers and look at the published rating curves instead. Those will, once you understand how to read them, tell you far more about how the motor will perform and how to drive it.