M.D.
I think you should pay her the same regardless of how many kids are there. She is giving you her time and expertise to 1, 2, or all 3 kids at one time. When she only has 1 kid, maybe she can do some light housework or something. But definitely do not limit her pay based on how many kids were there. I would not work for that. I nannied before and whether it was 1 or 2 kids, I was paid the same.