Before you go to extremes, make sure that the spark plug in cylinder 3 is functioning correctly.
I had the same thing happen in my old car, and I took it in to have it serviced.
They told me that one of my cylinders wasn't firing, and that the piston could have snapped off inside of the cylinder.
I went home, and found out that the spark plug was cracked.
But yeah, if they did do diagnostics, then they would have already checked the plugs, and then your engine is most likely done for.
But the shop I took it to was a shitty hole in the ground, and they didn't bother to do proper diagnostics.