2-man mechanics

Question
Bases are loaded with no outs or one out.  Base umpire is in position three where he should be.  Fly ball hit to right field.  Who's responsibility is the runner tagging at third base?  I have heard both the plate and the base umpires responsibility.  Would you please clarify the proper mechanic?

Answer
Hi Bryan,  "by the book"  the PU has responsibility for R1

The PU should move from behind hp, line up R1 and watch for the tag

The BU moves inside, bh, take the tags of R2 and R3 and be prepared for any play at 2nd.

If we deviate for some reason, we COMMUNICATE!

Mark