Prerunner is right you need to look into head gasket/cracked 0331 head. See if there's any milky looking stuff on dipstick or oil cap, what did the old plugs look like? Pull 2 and 3 back out and look at them again. Do a compression check if you got a tester and go from there. Between 2 and 3 are were they are were 0331's are most likely to crack.