Jim, you failed to mention when the boards blow? what is the power delta, verified a non-floating ground or a systematic look at all the wiring for solid connections or fraying or the best one - is it the correct part number. The assumption is that the first board blew for a reason and the next two are probably for the same reason. This has normally fixed 95% of all my CNC equipment in the last 40 years. It is the simple ones that will eat up the time. Question? when the first board blew did it send a surge to the other boards and have you checked for staples or paper clips in the ribbon connectors?