I can't find any reference for this either, and I never play with Muggins, but I think the answer is fairly clearly your option 1. I'll go through the thought process.
Cribbage is basically a race, Muggins is a penalty for sloppy play and missing your point count. In a two player game, the penalty takes the form of your opponent getting the points instead of you. In a three player game:
In scenario #1
The player who missed the points is still "behind" by the same amount whether one or both of his opponents get the points. So it doesn't matter much if both players get the points.
In scenario #2
The player who missed the points is penalized more if only one opponent notices compared to if both opponents notice. Why?
In scenario #3
If only one opponent is awarded the points and gains the advantage, even though the other opponent also noticed, the other opponent is penalized as much as the player who missed the points. Why?
Unless there are good answers to my "why?" for #2 and #3, I think the only fair way to call Muggins is to award both players the points.