Hi Acuario, and thanks for your thougts!
The unit I had in mind is an inverter with 4,5 kw max heating capacity. I dont know it's lowest capacity, but I've seen some inverters around 5 kw has lowest from around 600 watts, so I would guess running on one of three evaporators should go just fine.
Thanks for the reminder of solenoids too, I googled "3 evaporators on one compressor" and found some nice explanations on liquid line solenoid valves and thermostatic expansion valves.
So it looks like that part of the project is doable. But I still need to find a way to control the variable speed of the outdoor unit, without the electronics from an indoor unit.