Not necessarily relevant (and difficult to look up). I might also ask how many times this year have the Sox used 3 catchers in one game.
But in the post-season, teams typically go to their bullpen earlier. At that point in the season, a lot of pitchers have thrown their share of innings and might not be as effective as they were in April and May. Certainly starters are more worn out. And if there is anything to Sale's injury, it might not be so easy to pencil him in for 6 or 7 innings.
If the Sox have a reliever or two who is not 100% or simply not having a good week, the more options to replace him the better.
Don't overvalue the pinch hitting. The benefit of pinch hitting for, say, Leon, really only gives you a slightly better chance at getting a necessary hit. And really, how many times per game will they be doing so? If it comes down to keeping Christian Vazquez (clearly not a good pinch hitting option) over, say, Bobby Poyner, I'll take the situational lefty in that case and take my chances of stopping the opposition...
Really, the only advantage to keeping 3 catchers would be if the Sox have an injury at the position that necessitates removing the catcher from the game. While that has happened