360 LA getting oil in #3 cylinder.

it very well could.
Blow-by should be entering at the front under the throttle blades. There is a splitter passage there that dumps it into both sides of the Two-plane intake, and it gets enriched by the fuel coming from the Idle and transfer circuits. Those blow-by gasses are NOT all combustion gasses especially not at idle. At idle they are mostly atmospheric gasses, so IDK, 21% oxygen . So when the system is hooked up right, it's gonna lean out the primary fuel.
But when you stick it on the booster port, that gets sucked into the back 4 cylinders, which are 5,7,6,&8 and they go lean. This is a hard tune, because the front 4 are rich, and the back 4 are lean; so I just never do it.
However, IMO, this should have nothing to do with the oil-washed piston top. Have you done a compression test yet? My guess is that the fire-ring blew out and now oil is coming in thru the hole. That happened to me once. Compression test found it rightaway.