The way I see their relationship, based on the subliminal message I thought is beeing shown in the anime, is going like this: Umi loves Kotori, Kotori loves Honoka, and Honoka likes both of them.
I uses like here not because I think she's in love with someone else, (I can't see anything between Honoka and Tsubasa >,<) but because she's either too dense about love or too focused on the whole school idols thing to be concerned about it. Of course I do agree that she likes them enough that she might agreed if one of them ask her out. And since the one who loves her is Kotori, I'm not with HonoUmi faction.
I do see it's possible for Honoka to be with Kotori, but it also depends on whether she noticed Umi's feeling for Kotori or not. She might be too dense to notice it and agreed to going out with Kotori. But in case she noticed it, she won't think twice to shot it down. She'd do this because she'll put μ's harmony in priority.
And that's what might actually happened in that episode when the three of them in triple hugs, maybe Honoka noticed their feelings, and that is Honoka's way to say "Both of you are important to me, so (for now) let's keep how things are like this". Well, a kind of "let's just be friend for now" rejection for Kotori, and most likely even before she confessed her love. Well, I think Kotori herself is content with it.
As for Umi and Kotori, I believe Umi always knows Kotori's feeling for Honoka. This might or might not stop her to ask Kotori out. If I'm to imagine Umi confessing her love to Kotori, eight out of ten she'll accept it. So KotoUmi is possible.
My logic can't accept HonoKotoUmi no matter how sweet it might sounds like. It fit them, and "everyone can be happy" that way, but there's now way I can imagine a three way relationship like that without leaving each of them favoring one over the other. Well... let's just use "love can't be explained with logic" and "this is an anime, so let it happen" to keep enjoying the sweetness of this ship can offer.
Gosh, I wrote too much. It's not like me at all.