I have 2005 trailblazer ls 4.2 I6 how can I troubleshoot my intake and exhaust valves?

I have a check engene light on, did a dianosics an got a cylinder 3 misfire code i replaced the coil pack, all plugs and still get the same reading i took it in an I have low comppression in cylinder 3 all other cylinders are running above 150psi cylinder 3 is running at 75psi allI notice is that when its started lights flicker alittle like its losing power an it shakes as I let off the brake to drive once I start driving it dont shake anymore I figure its my valves intake or exhaust valves is there a way I can trouble shoot this problem to narrow down what the exact problem to this is