Calculate CR of creatures that share spells (like a hag conven)

I recently got inspired by this stat block, that shows that witches can get more powerful when together.

I want to make a similar creature that gets access to some spells when 3 or more of them are within 5 feet of each other. On their own they are CR 1/2, but together they can suddenly pack a punch. I guesstimated them as CR 1 now, but I wonder if that is correct?

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