Have you tried the teaspoon of oil down the plug hole of #3 to see if the compression came up at all?
Might be worth investing in a cheap boroscope (usually abut a tenner, plug into a laptop/tablet/phone) to poke into the plug hole and see if there's anything obviously out of place.
Not sure i understand what you've said about the valve clearances, i can see the correlation but the larger clearances correspond to the higher compression which is what i would expect.
Would i be right in assuming that all plugs were out for the test and only the tester was screwed in to the relevant plug hole for each test?
Nearly forgot - did you remove the hushers before checking the valve clearances? As many will tell you on here, they can cause false clearances to be ecorded if left in place.