Running down a cylinder

Have checked the plugs and wires. Getting spark out of both ends and even replaced the coil pack. So its getting spark. 2.5 months ago I put an injector cleaner in the vehicle. Now the vehicle starts up and runs but it is very rough. Seems like it is starved for fuel in the 3rd cylinder. I say that because when you remove 1 2 4 the engine almost stalls. But when you remove the third plug it still runs the same with no effect. Which means there is something going on in that cylinder. I did the long screwdriver handle on the injectors trick. They all sound similar but its slightly less noisy on the problem cylinder. My fear is that it is valves or something internal. Which would mean its death. Is there something I can check to measure for internal damage. Such as a compression checker? Is there anyway to check the valves. One other side note the spark plug was pretty nasty coming out of that cylinder and it had alot of gunk on it.