I feel like Poly or Throuple isn't quite the right way to describe what these three are.
Amane and Ijichi don't come off as romantic to each other almost at all, more like a BFFs/sisterly bond.
So it's more like a "Love V" or "Love <", but like with bold dotted line on the open end?
I'm just being technical but for brevity's sake Throuple still works just fine s it seems to have be the most used term in the comments for awhile.