I have a Trane XE 90 furnace which is in the second heating season. I've just recently had the same problem with getting the code flashing 3 times. After having someone not associated with a Trane dealer troubleshoot my furnace, and finding no problem with any of the usual switches or diaphragms, we were at a loss, thinking the problem must be in the circuit board when we discovered the problem. The problem was with a connecter at the upper left hand side of the circuit board not being completley snapped in place on the left side. It had been arching and had finally got to the place where it wasn't able to make contact any longer. It looked as though it had never been completly seated from the factory. It seats on both sides and is fairly robust in it's seating as it took a little effort to get it to snap in completely. After cleaning the contact of both the male and female at the far left side of tne connecter and putting everything back together, I've no other problems and the furnace is working fine. I don't know if this will help anyone, but just thought that I would let everyone know of this possibility.