I'd say you have one of the three a blue layer and the other two are green layers. It's possible the blue layer is only laying a couple of eggs per week while the green girls, besides being two of them, are laying more per week per hen.
I'd agree with your idea except that other than that first possible blue egg, all the eggs they layed last year were green. And once they all started laying, there were many days when I collected three eggs from them. That seems to indicate to me that they were all laying green eggs. Unless it's possible for one hen to lay two eggs a day?
Thanks.Beautiful eggs!
My EE lays green tinted and end of season, eggs are light grey almost white. It happens. Usually it s a shade or so off. If it is indeed the same hen laying I'd guess a few will be bright blue until her brown "coating" gets rolling again and they turn shades of green.

Well, one of the girls was just in the nest box, and now I hear the egg song out there, so I guess I'll go see if they've left me anymore eggs to add to the collection.