D..
I don't think that her friends would understand why she's hurt. She introduced them. They would think that she wanted them to be friends. If she tells them that she is hurt not to be included, they could think that she is needy and clingy. I doubt that she is, but they could perceive it that way.
She should continue to do things with them separately like before she introduced them. Then maybe at some point, all three can do things together. If they feel like she's upset at them for pairing off, they may not feel comfortable seeing her again, and that would be a shame.
Dawn