Without the electronics from an indoor unit you will have to build your own controller for the outdoor unit as the chances of breaking the communications protocol with only one end is pretty impossible. Even with the electronics from an indoor unit to allow this it is a challenge - read my thread on
controlling a DC inverter machine
With the extra complexity of 3 machines and the need to variably control the output to balance across the 3 you have a big challenge.
ThMihov is tackling the use of an inverter machine in his project
My DIY heatpump attempt but has the internal unit electronics to hack with an IR Blaster rather than attempting to replicate the control system.
Have a good read and search for BLDC motor control circuits as this is what you will need for the external unit.
Good luck if you decide to proceed and keep us posted!
Acuario