Check compression from cylinder 1-2-3
When compression in cylinder 3 is low, take in some ml of oil and repeat the test on cylinder 3.
When the result is better
example without oil : 3-6 bar
with oil 10 bar and the "same" as cylinder 1 & 2
The Piston rings are damaged
The special tool to check this , have a low budget. the expaned test for compression loss also low budget and then you can test the valves ( in and outtake )