What association are you calling for? May make a difference...
Let's assume no one goes out to call the catch or fair/foul, and let's assume we're talking ASA.
If there are no runners on, 3BU goes inside to cover the batter-runner going to 2B. If there's a lone runner on 1B, 3BU covers 2B, including the BR. If there's a single runner on 3B, 3BU releases to cover the BR going to 2B only if the runner on 3rd is absolutely committed to heading home.
So in a nutshell, as long as 1BU is NOT counter-rotated, 3BU will usually have calls at 2B.
In all other cases, 1BU has the BR going to 2B.
If I screwed that up, someone will let me know.
