Bizaro, this is taken from cgminer:

GPU 0:  63.0C 3519RPM | 633.0M/623.9Mh/s | A:28539 R: 91 HW:0 U:  8.69/m I:13
GPU 1:  67.0C 3471RPM | 590.2M/587.0Mh/s | A:26510 R: 54 HW:0 U:  8.07/m I:10
GPU 2:  66.0C 3481RPM | 635.1M/626.0Mh/s | A:28425 R:103 HW:0 U:  8.66/m I:13

GPU 1 has lower rates because it is underclocked to 1150 MHz core and a little bit undervolted as well (Trixx voltage set to 1200 and GPU-Z is showing 1.144). I did this because it is located on the mobo itself and can not get very good cooling. The other two cards are raised with PCIe 1x to 16x risers and they get better cooling.

At the moment we are mining a single block on Slush's pool for almost 5 hours now. The worker statistics show 1851.842 MHash/s (all three cards) and total 7598 shares for this block.

When undervolting just test if the cards are stable. For instance when I was testing mine, if a card was not stable, it was restarted (the monitor blinks and looses the clock values). You can also test with 3D Mark or some stress tests. As if the mobo is fried, unfortunately I am not sure how we can really test this without putting the cards on different motherboard. However what PSU are you using? It would be easier if you can try to replace the PSU and see what is going on. I generally do not believe in coincidences but just do not see how this can happen from the undervolting.