What could cause my in-wall air conditioner to blow 3 power boards?
There is a distinct possibility that another another component of the AC unit has some how gone bad and is overloading or injecting bad signals back into the control board and in turn burning it out.

It would take local diagnosis - along with a service schematic of the unit - to be able to isolate the faulty component. But your repeated replacement of the control board pretty much indicates that some other thing as gone wrong with the AC unit.