Perhaps I'm commenting on the obvious here but #3 cylinder being 25 PSI and with an oil fouled plug argues against only a valve guide leaking oil. Oil coming from guides tends to support compression pressure even on a worn cylinder. Nos. 4,5 & 6 are relatively better but still way low. For cylinder 3 I'd bet on a combination of burnt exhaust valve and, given the rest of your report, broken rings or a burnt piston. As others have said prepare for an out-of-frame overhaul.
It is possible that you have burnt valves and guides leaking oil but the rings are OK; a leak-down test might help diagnose that but you have to develop enough pressure to test the rings and badly leaking valves won't do that. Some other questions: Does it have a lot of blow-by out the breather(s)? Does it hold 10-15 PSI hot and idling? or is it more like 0-5 PSI? Do the rods knock with the engine floating (no load nor overrun)? Do the pistons slap when cold? Can you hear that low-pitched thump of a worn main bearing? How much oil does it leak out the crank seals?
If there's little blow-by, its reasonably quiet, holds good oil pressure, maybe you could get by with a head job. I have seen these engines with badly burnt valves but little else wrong but not often.
