Don't know if all three axles have brakes, but if RoyB is correct about each magnet drawing 3 amps, then even 3.3 ohms would be high. But, you're right; the controller should do SOMETHING. It sounds like you have a factory controller since it is a proportional control and not an inertial type.
So, if the controller is "doing nuttin'", do you mean it's not recognizing the trailer? If not, then maybe it's just a bad connection at the trailer connector at the back of the truck (?) - common problem...
If you're sure the trailer connector in the truck is OK, check for water in the connector. I've heard where just a small amount of water intrusion will mess up some of the factory controllers. If you have a "Y" connector under the truck to provide for a bed mounted connector, don't forget to check there for water in that connector.
