For the threesome, you can be in relationship with Ian but Lena has to be mad at him for kissing or sleeping with Holly. If he didn't go to the trip and Holly didn't kiss him, then yeah, being with Ian will not let you to participate.
Personally, I think the threesome shouldn't have so many restrictions because of the completely different mood of the scene, similar to the one night where they casually kissed. And Ivy has sex with Holly anyway, so it's not like she'll ruin anything by joining. Her main complaint about Ian hooking up with Holly was her opinion that Holly is an innocent girl that can't handle casual relationships. But after seeing that she's very willing to follow Ivy's instructions through, she should realize that Holly is not that innocent anymore and she doesn't need babysitting.