Spark plug wont't come out in the #3 cylinder. What causes this ? Plugs are regularly replaced on recommended schedule.

Soaked overnight with liquid wrench, will tryvagain today.

Usually over torqueing them will cause this. The threads get galled and it kind of welds itself in place. Leave it. You know you didn't need new plugs anyway...

You do use an anti-seeze compound, don't you? Just be careful that none of it gets on or very the electrode. It's conductive.

Did you get the plug out? If not, get ready to buy some new tools. If your lucky, the plug will just pull the threads out of the head and leave you with a hole that needs a time-sert or heli-coil installed. If your real lucky, the plug will break off and leave the threaded part stuck in the head. Sometimes you can tap the tang end of a metal file into the old plug and twist it out with a big cresent wrench tightened down on the flat of the file. If you try this make sure the valves and piston aren't interfering with the file in the combustion chamber. Good luck!