[Andy]

I'll agree with Dave here in that it is a general rule of thumb, but there are exceptions. i don't see the play described in the OP as an exception, but would say to handle it pretty much as Dave described. The key is that U3, who has chased has turned his eyes back to the infield to be able to assist if needed.

I remember two times in 3 umpire games where I have chased and have had to either come back or should have come back to make the call in the infield...

1. I was U1, no runners on, ball hit to right-center gap. I chase, ball is not caught, F9 picks ball up near the fence and throws to second as the batter-runner attempts to advance to second. I see that neither of my partners is anywhere near second base. I start busting back in, have a decent angle but am still far away and see the runner beat the throw. I make the big safe call as I am closing in.

2. I am U3, runner on first, one out, blooper hit back behind second base. F4 and F6 running out, F8 charging in, ball falls between all three of them, I have chased for a possible over the shoulder or shoestring catch. Ball is picked up and thrown to F1 covering second. R1 had to hold near first in case of a catch and the play at second was close. U1 was unable to get inside due to the presence of F3 and R1 and ended up making the call from the baseline directly behind the advancing R1. Feedback from the UIC later was that either I or the PU should have read the difficulty U1 was having getting to position and moved to make the call at second from a more credible position.