This is not a unique situation first of all. Second of all, please put some paragraph breaks in your post before the 12 hour window to edit is over. Its really hard to read.
Sounds like its tiime for boundary negotiations my dear. If I were you I would sit down with them both and tell them all this. Work out a schedule that means your partner is home alone with you for several nights a week, out at her girlfriends for several nights and all three of you spend time together once a week after which she then goes home alone. Maybe your partner should have a night to her self in there too. She is her own primary after all. She needs to look after her self too.
I would suggest no contact during the nights you and her spend together except maybe once to touch base and that's it. Its time for your partner to balance her plates I think, if she is to be shared. The two of her gf's need to help her with that by being good hosts to each other, respecting the boundaries laid out and having integrity.
I live with my partners and we have it down to a fine art after several years. It seems that everyone goes through this stuff and eventually NRE needs to be put aside for consideration and a suitable division of priorities. Poly people don't get to indulge themselves in NRE when they have more than one partner. It just isn't fair to the others. If they wanted that they would be monogamous (and be childless, they are effected by NRE too).
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