3 Person Mechanic Question

[rwest]

With runners on 1st and 3rd. 1B U is shading the runner at 1st. 3B U is on the line at 3rd. The mechanics section of the rule book does not adequately cover the responsibilities, in my opinion that is. I understand the responsibilities when either of the base umpires go out on the fly ball. But what happens when the fly ball is hit close to the foul line in RF? The mechanics says that the plate umpire is responsible for calling fly balls when the RF is going toward the foul line. This means that 1B U does not go out on the fly ball. Nor does the 3B U. Who has the tag up responsibilities? Where do the base umpires go? Where does the plate umpire go?

My guess is that both base umpires button hook in. The 1B U takes the tag up at 1B. The 3B U positions himself at 2nd, maybe(?). The plate umpire has the tag up at 3B and the play at the plate and 3rd base.