missing compression

now i had a turbo m50 engine 6cyl 2.8 liters and i found a missing power on the engine , i made a compression test and i found all cyl. have between 12-13 bar except cyl 3 has 9 bar i think the problem is in the rings or in piston ring lands, should i change the piston or rings for the effected piston or change the all 6 pistons ( to make sure the miss comp. from where the head , valves or piston i drained some oil and measured and the gauge rise so now i pretty sure its from the piston or rings )

The first step would be to investigate further and find out what is causing the low compression. It may be the result of a damaged piston or poor ring seal but it could also be a valve that isn't seating correctly. You can either strip the engine to inspect more closely, or you can perform a leak down test to give a better idea of exactly where the cylinder leakage is occurring.

Once you know what is causing the issue you can decide what to do about it.