[SanDiegoSteve]

There is a NCAA interpretation from Dave Yeast which, while not an official OBR interpretation, is a good rule of thumb as far as I'm concerned:

"While a fielder may not block the base without the ball, a fielder may move into the path of a runner if he must do so to make a play, i.e., glove a throw."

And the NCAA play example which illustrates this interp is as follows:

Play: R2 tries to score on B1's short single. The right fielder's throw is errant, and the catcher moves up the line to grab it. He collides with R2 but tags him out. Ruling: The play stands. "Both players were doing what they should be doing." - Yeast.

In my opinion, F3 was doing what he should be. The BR, although entitled to do what he should be doing, should see that the throw is off-line (as F3 moving towards him off the bag should be a clue), and attempt to avoid colliding with F3, if he wants to avoid getting the wind knocked out of him.