low conpression on ONE cylinder

Here's the list:

head gasket

exhaust valve

intake valve

piston ring

Chose one!

How do you know #3 is low? If you can check the compression of engine cylinders, you should be able to deduce the possible causes.

How low is it? What are the numbers? Does it burn oil? Does it smoke? Mileage OK, or poor?

Carbon built up on a valve seat, or on the valve stem, can cause the valve to leak and loose compression.

A rocker arm that is adjusted too tight will do the same thing.

A valve keeper that has come loose will also do that.

A cracked head, or block will also have low compression.

A compression gauge that is not working correctly, or does not seal properly will show low compression.

Is the engine running poorly? What does the #3 spark plug look like compared to the other ones? How low is #3 compression? Have you done a wet compression test?

Maybe do a leakdown test to see where compressed air is going. You will hear the air leaking or see it bubbling in coolant with rad cap off.

Would help if we knew what it varies from the other cylinders. Placing a little motor oil in the #3 cylinder and redo the test. If the compression increases the problems in the rings. If little or no change its a head problem. Bad head gasket, leaking valves or head could be cracked.