I am also using 3 x Sapphire 7950 but with some cheap GB mobo. The voltage is very good (about 1.56 each) and I undervolted one of the cards to 1.119 in order to lower temperature (it runs stable at 1150/300 core/memory and the other two are @1200 core). This is running stable for almost 10 days now and do not have any issues. What you describe definitely looks like mobo/PCIe issue but I do not see how undervolting will burn the mobo. My personal guess is that it is coincidence that it happened after you undervolted one of the cards. Some motherboards can not constantly run their PCIs on higher voltage and over time the tracks are just burning. Since yours is with several PCIe x16 the cache for this to happen is small (the vendor should have done the voltage calculations) but my bet is still on this.