P0 308 replaced plugs coil swapped injecter and checked compression

Could be low compression on that cylinder. Do a compression check. Also see if there a vacuum source connected to that cylinder that may cause a miss.

start by replacing spark plugs and cables. If it has coil packs swap coil packs and see if the misfire changes place.If no change check harness function, if it continues have a compression test done, if it fails the test then so a leak down test to figure out where you are loosing compression

p0306?
misfire means, low combustion energy on #6

1. bad spark,
2. bad injection,
3. bad compression

there are tests for each one.

i always check compression first. that why i dont spend a fortune
guessing. 30min.,labor , done wide open throttle, 150psi min.

i also heck the fuel pressure doing the SOP leak down test. (fSm)
then the injector balance test at same time.
finds leaking injector and clogged.
got cops?
you could have swapped coil packs COPs to see if that cured mifire.
swap any other with #6
if problem follows the coil pack COPS, then its bad.
my v6 had a 3 coil pack and cant swap just one coil. all 3 or none.

the misfire is real. you can feel it and the PCM measures it.

"All cylinders have 120 psi compression." I would be seriously concerned about this. Typically, an engine's compression should be much higher e.g. 180 psi or more. Could mud have gotten into the exhaust system causing restrictive back pressure?

100 psi per cylinder is way too low--should be in the vicinity of 150-165 per cylinder
back firing indicates wrong firing order and as you have replaced the plugs etc check that you have the leads on the correct plugs in the correct order
the timing for the engine should be around the 10 degree btdc and not tdc

Spark plug(cracked, carbon tracking, incorrect gap). Ignition coil( inoperative, open/sorted, computer not comanding it on/off). Injector(inoperative, open shorted or computer not commanding it on and off). Compression(wash down, worn rings, belt or burnt valves...). Try swapping coils and see if the misfire moves to another cylinder if it does replace the bad coil.

Remove the spark plug of cylinder No. 3 and inspect it for any water or coolant traces on it. If water droplets are found, the gasket is at fault and need replacement. If not, the gasket is fine and does not need any replacement. It is recommended to check the valve clearance before entering into major repair.

remove the spark plug and check for black carbon tracking marks then while the plug is out do a compression test if no lines and has good compression swap the coil from another cylinder and see if the problem follows the col if misfire doesn't follow swap a injector and recheck

check plugs wires and distributor too.....should be pretty expensive too, cadillac is not cheap...

Check the wiring to the coil

Check to make sure the fuel injector is working properly.

Check compression on the cylinder