Serrated Arrows enters the battlefield with three arrowhead counters on it.
At the beginning of your upkeep, if there are no arrowhead counters on Serrated Arrows, sacrifice it.
, Remove an arrowhead counter from Serrated Arrows: Put a -1/-1 counter on target creature.
8/1/2008 The upkeep trigger checks the number of counters at the start of upkeep, and only goes on the stack if there are no arrowhead counters at that time. It will check again on resolution, and will do nothing if you’ve somehow manage to get a new arrowhead counter on the Arrows.