I had a UNIBUS backplane where one of the connector tabs popped out a bit, and the next time I put a card in that slot it mashed the connector down into the bottom of the block. No way to fix it of course. Had to mark that slot as DNU. So now I do a bright flashlight inspect anytime I am going to insert a card into a slot, even if I just removed it. So beware. Backplanes don't grow on trees these days, harder and harder to find.Hopefully he can fix the issue with slot 3 easily. Maybe just a pin that's stuck out. Or something wedged into the slot keep the pin from making contact. I'd look in there with a very bright light to see if there is anything that can be seen. Or possibly a bad solder joint on the connector? Seems less likely.