compression test results

[nova_172]

did a compression test on my 1.2spi after tuning it, befor it ran fine, i did the test because it was idling dodgy.
the results were
cylinder 1 170psi
2 170psi
3 120psi
4 170psi
i then poured some oil down cylinder 3 and tested it again and the pressure went up 2 170psi, so is it definetly piston rings, ive got the head off and everything looks ok, if i hange the rings is it best 2 change em all.
spec of car below.
thanks

[nova_172]

i did a compression test befor which read 160pis all round, i think its just a worn piston ring that is letting some by, also it doesnt slowly pump up 2 that pressure on the first crank it reaches that pressure.