I was in a three through secondary school and they can work, however at any one time two of us were closer. Not always the same two; we all had times of being the third one.
So, for example, if we were told to work in pairs in a lesson, we'd beg to be allowed to be a three, but if the teacher said it had to be a pair, then it would (at that point) always go the same way. Luckily for us there was another three in our form that we got on pretty well with.
We wouldn't have not invited each other to a party, but did always do things as just two if they happened that way (and not leaving the one who was currently the "third" out, even then it was probably fairly even between us). But we were always open what we were doing, and if the other asked to join in, it would have been accepted no problem.
I think what someone said about it really being two pairs: Mollie and Sarah, and Mollie and Poppy is probably closer to the mark. Particularly if Sarah is in isolation and perhaps can't do as much, then maybe Mollie (does Mollie still need to isolate etc?) and Poppy spend more time together naturally.
I'd wonder perhaps if Mollie's mum knows that they're growing apart and is trying to not have Sarah feeling left out, hence pretending they're having a pair chat for reasons where Poppy is the issue.
Would Sarah have been able to do the party? I'm just wondering if they were doing something active as you say she has leg splints and need a special chair. If she couldn't do the activities, then maybe Mollie would have also felt she had to stay with her, and then I can see that could have been awkward at Poppy's party.
I can see if that was the situation, then 11yos, who aren't known for their tact and thoughtfulness, might have thought they were being nice to call Sarah. Might they have done this a previous year if Sarah was isolating, maybe? That would mean in their eyes they might feel it was including her, especially if Mollie told Poppy that she'd come from Sarah's, then Poppy might have assumed she knew anyway.
Obviously Sarah does spent time with Mollie without Poppy too, because they were together when Mollie left. Does she spend time with Poppy on her own? Does she ever invite Poppy to things on her own?
If she doesn't then I think it really is as said upthread, two pairs of friends with a mutual one in common.
No, I don't think Sarah's being over sensitive. I think almost anyone would be upset.
But I also don't think it was necessarily mean girl behaviour.
I'd say you have a choice:
- Go nuclear. I suspect you'll then find Mollie goes off with Poppy. But you might not, although I doubt Mollie will drop Poppy either.
- As someone else suggested, love-bomb Mollie. Invite her to wonderful things, and leave Poppy out.
- Or try and see if Poppy will do things just with Sarah. If she feels she's really Mollie's friend that she sees for her sake, then that might change things to be a more equal three. Don't ignore Mollie, but encourage the Poppy-Sarah friendship as well.
I'd be inclined to try 3, because if Sarah has time off for isolation and can't always do things others will do, they're the age when they want a friend to be there as much as possible. So if you encourage the threesome then they will be there for Sarah when she comes back from isolation. Whereas if she's just friends with Mollie, then if she's off for a long time, there's a reasonable chance that Mollie will have moved on and not be interested in Sarah coming back.
That was one of the best things about being in a three, I found. You always had the other one if one couldn't make it/was ill.