2015 Silverado 4.3L V6

I just bought this truck from a dealership 3 weeks ago.Test drive was stellar, everything ran smooth and efficient. Second I threw it in park after the drive home I had a shake at idle and a check engine light. Light went away next day commute to work and came back later that day on the ride home. Brought it into oreillys to run a test and got a cylinder 5 misfire code. Swapped the ignition coil right away to cylinder 3 wasnt the problem. Replaced the spark plug wasnt the problem. Brought it to a local body shop for diagnostic and it was the fuel injector they said. Brought it back to the dealership for a free fix. Picked it up the next day and got the same damn issue. Im running out of solutions guys. Any advice?