Karabell raised the possibility of all three guys saving 10+ games, which has been done 16 times since 1969, the last being the 2005 Diamondbacks (Jose Valverde, Brandon Lyon, Brian Bruney) and 2005 Braves (Chris Reitsma, Danny Kolb and Kyle Farnsworth). Arizona finished 77-85 but Atlanta went 90-72 and won the NL East.
Of course, those weren't time-sharing situations, but cases where relievers pitched poorly and lost their jobs. Arizona eventually promoted Valverde to the role later in the season while Atlanta traded for Farnsworth.
But Royals manager Ned Yost could turn this into a positive job-sharing situation, rather than just deciding on one guy as his primary closer (that's still Holland for now). That would take a little creativity, something you don't see much from modern managers. If Herrera is really the best Royals reliever -- he is in my book, with that upper 90s fastball (here's an Insider piece on Herrera and whether age is important for closers) -- then use him in tie games in the eighth or ninth inning or one-run save situations, and use him for more than three outs if necessary. If he can't pitch the next day, you still have Holland or Crow available (not to mention Tim Collins, another good arm in the bullpen). If it's a three-run save situation, don't waste Herrera; use Holland or Crow if they're available.
In other words, manage the bullpen more like a manager in the '70s or '80s would have. One of the teams with three guys to record 10 saves was the 1976 Phillies, with Ron Reed, Tug McGraw and Gene Garber, all of whom pitched at least 90 innings with an ERA under 3.00. I know times have changed -- that staff used only 11 pitchers all season (and of those pitched just three innings) -- but with a deft touch Yost could make his bullpen a bigger weapon.