Is it fixed yet? Anyway it sounds like you have a shotgun tech, changing parts until he hits the right one. Ok, stepping back I looked at the service manual and it has excellent wiring schematics, did he have a service manual? I found the board on the Indoor unit doesn't have a 15Vdc OUTPUT. The indoor unit does have a 15Vdc INPUT supplied from the outdoor unit, which comes in through pin 3 of the 4 pin connector. This 15Vdc is then dropped down to 12Vdc and 5Vdc to run the indoor unit. The input 15Vdc is common to all three indoor units from the same source, so if the problem is the 15Vdc source from the outdoor unit none of the indoor units would work. Question, did he replace the fuse with the right size fuse or something bigger? Or did he jump the fuse? (better check.)

I recommend checking the wiring at the outdoor unit and the indoor unit for a loose or corroded connection at the terminal blocks. Brown wire, may be labeled CN wire 3.

If the HVAC guy finds a bad wire he may not say so and charge you for a part out of embarrassment or greed. FWI, the service manual has a troubleshooting ladder to follow and the first thing they say is to check input voltages, then check the terminal blocks for bad wiring connections. It doesn't say replace motor, then PCB, and replace PCB again, and if all else fails check the wiring.

Look for yourself, starts on page 69.

service manual