I unfortunately have experience with multiple engine failures and I have NEVER known or heard of any reason for the oil filler to be spouting gas (whether smokey or not) other than one of more rings having blown. If you take off the filler cap and there's a rush of pressure coming out that's similar to what you feel at the exhaust tip(s), in my experience there's no question that one or more of the rings have gone to mechanical heaven. It sounds like no. 3's spark plug is consistent with that, as well. This is not to say you should not do a compression test or other tests to confirm; I just think you may as well start adjusting the budget for pulling off at least the heads and cylinders.
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