I suppose that a lobe on the cam could be so worn it doesn't allow enough opening in the intake valve to let enough air in--but that is a long shot. Might want to compar how much the valve stems are depressed across the head to see if #3 is different. That is the only test I can think of without taking off the head.

Far more likely you will need to pull the head to see what is going on. If you have never done this before it is pretty straightforward--just need to take your time and do it right. Several people on this board can walk you through the process. You can then get a machine shop to look over the head and see if you have a problem. It could be a bad head gasket, bad valve, any number of things.

With the head off you might be able to see if anything looks off with the top ring on the #3 piston--bit of a long shot, but worth a try.

Let us know what you find out.

--
Tim--'72 ES, '90 240