Help with mechanics

[drh898]

Runners, first and second, batter lines hit to left, I'm BU in C position. When ball leaves infield move into the infield so I'm inside the bases and I turn my attention to batter-runner and runner on first. I release runner going to third to the PU. Throw comes in from outfield, is cut off as runner stops on her way home and attempts to return to third. Whose call is it?

I was watching batter-runner and runner on first going to second in case of throw to second. PU said the call at third was my call. Is it? Do I watch the ball or the runners or try to do both?