bb-ref has Team Advanced Fielding statistics in which they look at (among other things):
Single w/ runner on 1st
Single w/ runner on 2nd
Double w/ runner on 1st
Flyout with < 2 out, runner on 3rd
Flyout with < 2 out, runner on 2nd, then they look at Hold% and Kill% for the sum of those five categories.
For this season:
LF -- BOS/MLB -- Hold: 62.9%/66.0% Kill: 0.0%/1.8%
CF -- BOS/MLB -- Hold: 42.3%/46.6% Kill: 0.0%/1.3%
RF -- BOS/MLB -- Hold: 61.5%/47.2% Kill: 0.0%/2.5%
I don't think the throwing of B-B-B is always up to standard. There are problems with throwing to the wrong base, missing cut-off men, and particularly with Benintendi and Bradley a problem of keeping throws on line. While all three are fast and generally have good instincts for tracking balls, there are still instances where all three have trouble with judging the ball with relation to the fences. While Fenway Park does add its own foibles, not all of the misjudgments have happened there but I do feel that Benintendi has yet to become "Green Monster comfortable."
The question has been asked in this thread, Where would Margot fit? What could they get in trade for JBJ? And then you determine who fits better in LF/CF--Benintendi/Margot or Margot/Benintendi (hypothetically, of course).