Who gets the points when calling Muggins in a three-player Cribbage game?
Whomever calls it scores it. The rules at American Cribbage Congress do not address muggins outside of 2 player, but the wording requires that it be called before pegging it. Since, in 3 player, there is a dealer, a pone, and another pone, it's simplest to let whomever caught it score it.

Further, in 30 years of playing cribbage, in both 3 and 4 hand, whomever catches it claims the points. This is incentive to pay attention to opponent play and score calls.