[FMadera]

The R3, from what I've been told, handles substitutions, facilitates TV timeouts, and does a little "serving and protecting" of the R1 if needed. The first two are the primary responsibilities.

Another forum indicated that part of this was because to cut down on the number of cameras needed to cover the match, they put the teams on the R1 side, so the R3 handles subs from the R1 side of the court.