Sorry, I haven't read what others have said here, so there likely will be some over lap in what I say.
In a nut shell, I think your boundaries are WAY too restrictive at this point and its time to change them. Having sex all together, not communicating out side of the triad and all that just doesn't work for the long haul and just breeds co-dependence as far as I have seen. You are not three people in a unit of one, you are all independent people that have independent lives and relationships. You with him, her with him, you with her AND then all of you together.
It might just be that she loves him more. It might be that he loves her more than the idea of all of you together... this happens. A LOT. In fact almost always in triads. No biggy, just go with it and change the boundaries. It could be that you and her will not be intimate any more or less, and they will be more, who knows... you won't know until you let go of the control over this I don't think.
This is a very common situation. You are in good company. It is the usual scenario with unicorns coming into a relationship. They are there for a short time and then things shift. It can last if there is an adjustment of expectations, assumptions and boundaries that might mean a whole different type of relationship dynamic. Perhaps a vee would be better. This is where it usually ends up anyway.
Who knows, maybe things will work out if you put your foot down. I'm sorry you feel hurt. That is a tough one.
I suggest doing a tag search for "unicorns" "triads" "lessons" and "foundations" and anything else that looks interesting and see what you learn about others in order to see where you could go next with this.
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