[Secret Agent Man]

Does it have to be three concurrent batters or just three batters across 9 innings?

I mean what if the tean already has two outs and they bring in a relief pitcher? You can’t bring in a relief pitcher because if they get the next batter out there’s no 2nd and 3rd batter?

[Ouderkirk]

Once you leave the game you can not re-enter.

So, if a relief pitcher comes in, he has to face at least three batters. I would think that in the 9th, that you could replace at will.

THis certainly changes the makeup of the bull pen and how you may pull the trigger on a relief. You have 2 outs, and a man on 2nd and their slugger coming to the plate (and your pitcher coming to the plate in your next at bat) You yank the starter place in a setup guy for one batter make the out, and then pinch hit for him at the next at bat.

This rule precludes that. The pitcher has to come to the plate in the next inning because he hasn’t face three.

[Repeal 16-17]

I mean what if the [team] already has two outs and they bring in a relief pitcher?

It's 3 batters or a third out, whichever comes first.

[TBP]

The pitcher does not have to face three batters if the inning ends before his third batter.

[Verginius Rufus]

So no changes would be allowed after one out in the ninth inning.

Maybe there will be an exception if the pitcher is knocked out cold by a batted ball and has to be carried off the field.