Bent valve, dropped valve guide, dropped valve seat or broken valve. All not good. BUT...before you pull the head though, pull the cam housing only. Inspect that valve compared to the rest from the top. if it is very much higher than the rest it is likely bent or one of the other above scenarios, If it looks OK then try this:
Set the #3 cylinder at BDC for that piston (piston down). Spray carb cleaner into the intake port for cylinder #3 until it pools on the closed valve, then with a flat punch and a hammer tap the top of the valve stem to momentarily open the valve several times until the carb cleaner is gone into the cylinder. With your leakdown tester compress Cylinder #3 to whatever the top pressure is for your compressor. Spray carb cleaner in the intake port again and then cover the port with a rag. Then again with a flat punch and a hammer, tap the top of the valve stem. It will make a big bang as the compression escapes the cylinder. Do this several times. Then check the leakdown again. Often times it is severe carbon buildup or other foreign matter that jams the valve from seating. I cant tell you how many times this has worked for me.
Good Luck!