GMC sierra 5.7 only starts when you arc # 3 plug wire

Ok never seen this before and have no idea 2000 GMC sierra 5.7 will not start. I was checking for spark on the #3 plug to see if I’m getting fire and the truck cranked and ran smooth as long as I’m jumping it to the exhaust manifold. I tried it on all other plug wires no start it only cranks when you arc the # 3 plug wire. Also hooked the wire back up while the truck was running and the truck died no start until I jump the wire???? Any ideas I need y’all’s brain power thank you