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[danjreed]

A 2 axle controller is not made to deliver the current that 3 axles need to operate according to specification. Hence you don't know which ones will lockup.
The electric trailer brakes (excluding electric-over systems) can for our purposes be seen as resistors. They're actually electromagnets, and that creates some really strange things, so we'll just call them resistors for now.
Trailer brakes are wired in parallel. Meaning that the + wires are all wired together, and the - wires are all wired together. When 12V is applied to the + wire and the - of the magnets is on the - of the battery, all the magnets see 12V.
The problem is related to Ohm's law, V=I*R (Voltage in a circuit is equal to the current through that circuit mulitplied by the resistance as seen from the power supply).
When resistors are in parallel, the resistance that the brake controller sees goes down. In order for the controller to maintain 12V on the + wire of the magnets, it must allow more current to flow to the brakes.
Not having enough current will limit the maximum voltage that the brake controller can supply. Which is why it is unpredictable.
In order to supply enough current to the brakes, the controller needs to have components sized to handle the loads.

Unlike mechanical parts, electronics usually can't handle much more than what they are specified at. Obviously the controller manufacturer added a little security to their design since it still works, but I wouldn't keep doing that.