Here is how I understand it. Inductions motors have a certain 'slippage' which is by design. Permanent magnet motors, like the ones introduced with the Model 3, don't. The rotor follows the generated magnetic field exactly. Two of those motors don't work together as efficiently hence Tesla always combines a induction motor with a PM motor in all their current dual motor cars. The slippage of the induction motor compliments the 'rigid' nature of the PM motor. The rear motor is PM and does the majority of driving taking advantage of the higher efficiency. The front induction motor kicks in when extra power is needed. The dual motor version of the truck will probably have this configuration.
Tri-motor cars will be a challenge. Maybe Tesla has figured out how to run three PM motors efficiently together now. Or maybe they will continue to combine both motor types like they do now. I would think it might be difficult to have two PM motors on the back wheels. They spin at slightly different speeds unless you go perfectly straight (which is rare). Having two induction motors on the back wheels would fix that issue. The front motor would then be PM and would get priority most of the time in normal driving to take advantage of the higher efficiency.
Again, that's all my theory and is based on what people have told me about the different motor types. Once the first tri motor cars are out, we can only speculate.
