Would she maybe have eaten the shells?
I have picked her completely up to check a head count and pick up the shell remnants and only had two halves and 9 chicks.
I mean, I know twins are possible though highly improbable, I would think triplets would be impossible for sure-there's just not enough room, not to mention in the year I have had these birds I have only gottne one double yoked egg and it was out of one of my production reds, not the leghorns who I understand are very unlikly to lay double yolkers due to them being a more "commercial" bred, and that was the presumed egg they came out of.
Oh well, blessed mystery