I have a 1988 Isuzu Trooper with a 2.6 Liter engine it has a miss ?

when I pull the 3rd plug nothing changes all other plugs effect the engine the plug has fire and for safety's sake I replaced the plug I'm told the head gasket was recently replaced and that the problem may be the valves are improperly adjusted I seem to think it may be fuel related I'm trying to fix this for a family member so any input or thoughts would be appreciated