Why a miss-fire after changing Cyl#3 ignition coil and plug? Cyl. head problem?

Get a spark checker and make sure the coil can fire it. If not you could have a PCM or wiring issue causing the loss of spark that may simply have been disturbed when you did the first attempt and temporarily fixed itself. Without testing to see what you are missing, spark, fuel or compression you could have a really tough time figuring this out.

Yes. The intake valve doesn't seat properly causing a miss. I see it more when it's cold and extended idle. The only fix is to replace the cylinder head