Which linebacker is the pass rushing linebacker in a 3-4?

Whichever outside linebacker is on the quarterback's blind side. If the defense is playing a zone blitz, then both outside linebackers are blitzing and the middle linebackers are dropping back into pass coverage.

The outside linebackers usually.

Generally, the better pass rusher is on the right side in a 3-4 or a 4-3 because that's a right-handed QBs blindside. But, there's no rule that says a coach has to do that. Michael Strahan played on the left side and he's 6th all-time in sacks. Different teams have different systems. Some teams line up their LBs on the same side all the time and some have strong and weak who flip depending on where the TE is.

So, the answer is generally the ROLB is the best pass rusher with the caveat that it can vary depending on the system.

Could be any of the 4.

outside weak side

Whichever one(s) the coach says. Or none.

I guess it's the one who's the closest to the 4