I think you should just tell H and Y how you feel about H. And also tell Y that you care about her too, and completely respect her relationship with H, and don't want to cause problems for them in any way. But your feelings are there and you don't want to hide them any longer. See what they say. The worst-case scenario would be, H ends the relationship because you have feelings for him. But if that happens, then you will know for sure that he can never give you what you want from him anyway, and you can walk away (hopefully) without regrets. Trust me, what he is giving you now (sex without emotional attachment) isn't so great, nor is it hard to find. I guarantee you could find even hotter sex with someone else who doesn't have any emotional attachment to you, without half trying.
H and Y may be in a nonmonogamous relationship, but do they know anything about polyamory? If not, might it be possible to educate them a bit? A lot of people really have no conception that it might be possible, and acceptable, to love more than one person, and some of those people are definitely capable of it, and might open up to the possibility if they were clued in. I lived monogamously for years, not because I really believed in it wholeheartedly, but because I had no idea there were other options, and would never consider cheating on my husband. Once I learned about polyamory, I knew I was poly. Before that, I had no idea.