Have you tried demagnetizing the heads? I've had a similar problem with a regular cassette deck, where one channel would be out (or a lot lower in volume) compared to the other. Demagnetizing seemed to do the trick. If you haven't done that, it's worth a shot.
Also, how old is your 4-track and how much have you used it? I have an old Tascam Porta 03 that recently died - one of the belts inside had finally had enough, so it plays back really SLOOOOOOW. Not worth fixing, as I could buy a new 4-track w/much better capabilities for not much more than it would take to get the Porta 03 fixed. Besides which, I've gone digital. Anyway, my point is that, if your 4-track is pretty old and/or well-used, something in the Track 3 line might have quit on you. Hope not, but if demagnetizing the heads doesn't fix the problem, you might have to take it into the shop and have someone take a look at it.
Hope this helps...
