I confirmed a head gasket leak by sucking coolant out of cylinder #3 with a turkey baster.
Facts. Only cylinder 3 is affected. P0303 misfire on cold start. Exhaust steam for a few minutes until warm. It's accumulating about 1/16th of an ounce of coolant every 24 hours. The coolant loss does not move the indicator in the expansion tank. Block Tester indicates zero carbons in the cooling system. Dipstick indicates no coolant in the oil. Car is driven only 2,000 miles per year. I just finished a 500 mile Interstate road trip with no problems, and the coolant temp was steady at 99 degrees C (210 F).