Whats the cause for a misfire cylinderl#3 po 301

P0301 would indicate a misfire at cylinder 1 If you're near or have missed the spark plug replacement interval, I'd start there.

Misfire can be caused from low compression, lack of fuel or no spark. Need to have it checked out. How's the coolant smell. Does it smell like exhaust fumes? Then you may have a blown head gasket and the coolant is getting in the cylinder and causing the misfire. We seem to a lot of these problems. When were the plugs changed. Over 80K then at least pull the easiest plug to visually inspect it. If your plugs are very old replace the wire set too. I know they're expensive but better to do it now than get cheap and then have to spend it anyway after the plugs are installed. GL