As far as I can tell from the details you have provide, the problem started after you have replaced the PSU.

The 2nd unit which did not work was exactly the same p/s purchased at the same time.
Did you changed any hardware (ram, card(s), hd(s), dvd player(s) etc etc) after purchasing the pc? If so, what?
The 3rd unit is another brand but from an identical PC known to work
That doesn't mean that psu can deliver the same wattage. So again... Did you check the wattage on the original psu and the ones you have tried to replace it?