I have considered getting a apw7 unit, but I cannot determine if that would fix the issue or not, I currently think not.
The APW7 puts a decent 1.8KW at 230-240v, so the max you can achieve on a 110v would be under the 900w mark which is not enough to run 3 hashboards at least not on the default settings.
Is there any option to running the third board off of a second PSU from another outlet?
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.