You never said why the cylinder was replaced. Did it have the same problem before. Yes there is pressure, unless you run without a radiator cap. The pop off pressure probably is 15psi. Where the cylinder bolts to the crank case there is no demage, such as from a broken piston you forgot to mention from the last cylinder replacement. Is this a 3 piece base gasket design or does it have 1 gasket. If you have to remove all three cylinders I would draw file the deck of the crankcase. Then use a feeler gauge and a straight edge. By a straight egde, I mean a machinist square or ground stock. If there is any high spots, by "draw filling" you will once again have a flat deck. Then check each cylinder with the straight edge. If the gaskets are thin, like most of them, you could use two base gaskets under all 3 cylinders. If they are more then .010" thick then i would not unless you have access to a lathe or mill. The more you drop the piston in the hole the more compression you will lose and also you will have more squish clearance which could lead to detonation.