Holly is basically the honourary 3rd protagonist, the most to grow and change besides Ian and Lena. And she's sweet, hot, and super nice. I definitely was hoping the throuple was a possibility way before it was confirmed.
I could see Emma being added, but maybe only as a sometime member? I suppose there could be some devil throuples that reverse the sweetness, love, and light that is the Holly Trinity. But I don't really want to theorise what those may be. Though I guess, for cuck fans, cuck Ian and Lena is a throuple, but the 3rd is just whoever Lena wants in the moment.
Ian/Alison/Jeremy could be a faux-throuple I guess. I don't think Louise would be able to handle a throuple, she'd always want more. Ivy would stamp over everyone's feelings, but that's no surprise since she can't do any commitment.