3 man mechanics question

[pollywolly60]

High School play off game - three man mechanics.

Single runner at third base. Infield hit to shortstop. Bad throw, ball gets away, runner rounds first and starts to try for second. Runner at third is still holding between third and home to see where the ball will be thrown.

My question - whose call is the possible play at second?

As it played out, I was 1st base umpire, and I took the play at second because my third base umpire was holding at third with the runner at third. My UIC says this is correct,, and I've never known him to be wrong before. I just thought that possibly the third base umpire should have been coming in to cover the possible play at second, with plate umpire coming up to take the runner at third.

Comments....?