It's a really complicated situation, because she feels like the newcomer in our relationship but he and I were never officially dating, so there were no agreements made before they started dating each other. Now anything I ask for as a concession to my feelings is a direct negative impact to her, and so I'm uncomfortable asking for anything.
So you sit down in trio and make the agreements now
that are necessary so that all players are able to get along in harmonious manner. Time management, calendar, etc.
Clearly it's not working for you as it is in a loose structure. So articulate your needs and see if they are willing to meet them. Maybe they have needs of their own going unmet. Air it out and sort yourselves out.