I agree with WD45 but that should affect two cylinders - both 3 and 4. Plugs do occasionally completely fail. So do distributor caps. There could be a carbon track in the cap which redirects the spark to a different plug or ground.
I have the same problem on a Ford 901. It is #3 also but I assume that is a coincidence. I've changed plugs, caps, rotors and wires. As yours, #3 has the best compression. But it will not fire at idle - high rpm yes. I'm about to pull the valve cover and check the cam. I'm thinking it may be gettings an incomplete charge because of a flat cam lobe. It is hard to check the cam on a Ford 901 because of a lot of disassembly. A WD-45 is pretty easy by comparison.