I don't see why not, as long as every hashboard is connected to just one PSU, so every 3 6pin going into a hashboards are from the same power source, and I assume you will need to power the PSU that does not power the control board first.
Sorry - clarification: do you mean that all three hashboards must run off 1 psu, or that each hashboard separately must have at least one PSU? (meaning, I can't use two PSUs on a single board, which is what I think you meant...)
I guess from a simpler standpoint: I have two boards functioning now. If I get a second PSU and connect it to the third board via a separate outlet, that could power the third board and get all three hashing without needing a 220v outlet, correct?