Furnace does not start in heating mode. Tranformer is humming and the furnace makesa click and the led light shines solid and then flashes 3 times. We replaced the pressure switch and the same problem exists.

You need to check the fan switch and the fan motor. One is bad, most likely keeping the fan from running. The three flashes is the hi-limit tripping. This may help: https://dl.dropbox.com/u/8010163/How...e%20Manual.pdf
You will need to get a meter and learn to use it. Just changing parts does not work, never has on modern units.