Tbh I don't think that Hiroi becomes unsatisfied when his partner becomes equally interested in Take like they previously were to him, I think the problem is that their previous partners started thinking that Take would choose them over Hiroi if they say that they are 'just in love' or 'more interested' in him than Hiroi . Because from what I remember Hiroi and Take kept telling them that they're basically a packaged deal (you won't/can't have one of us if the other won't be there) but the previous lovers either didn't really care/acknowledged that or they started getting ahead of themselves that Take would choose them over Hiroi who's been with Take from the start (that's why none of them would be satisfied at the very end because no one would be getting what they 'want'). Also Take says that all their previous partners were always just Hiroi's that he shared with Take because that's how they've always been, they share everything together. The problem is that their partners don't 'share' Hiroi and Take too (idfk it's like what Mine says when they had the argument) and ending up being more bias to Take.
That's why I also think their relationship now with Mine is better. Like you said Take and Mine consider Hiroi as their center. Mine ends up understanding (I think) that Take and Hiroi cannot really be separated so he and Take start getting along more (???). And while Hiroi notices this and brings it up to Take, when Take echoes what Mine said when they had an argument sort of put Hiroi's worried mind at ease about Mine choosing Take over time. That's why I like to think that before the extras, that all three of them had some time to actually be comfy around each other that they would have no doubt of choosing one over the other, even if only two of them are being intimate while the third isn't there.
TLDR: Idk man I just want the three of them to be happy with no problemss ╮(￣▽￣)╭ also kinda an incomplete thought cus I reread this manga when I just woke up (￣∇￣")
Rereading this I still think it's very good
They didn't start off very well (Mine for sure was not ok with take being there)
But their relationship developed really well
I think what people are not getting is that Take and Hiroi's relationship isn't romantic, but that doesn't mean it's not deep, on the contrary, they're so close they push the boundaries of platonic
Meanwhile Hiroi and Mine's is 100% romantic, they're super lovey dovey and cute
And Take and Mine's is primarily sexual, they have great bed compatibility and later on get on better terms in general
I really enjoy this bc they work very well together
Sure sure, it's kinda fucked up how Hiroi wants to share everything with Take, but becomes unsatisfied if a lover becomes equally interested in him
And Take himself is so attached to Hiroi that he'll completely lose interest on someone that isn't revolving around Hiroi as much as he does
And THAT'S why Mine is actually perfect for them
He and Take agree that Hiroi is their center, but know that one will never overtake the other or choose each other over Hiroi
Idk, I just find how the three of them fit together so cute
They seem fucked up, but to be honest, they ended with a very healthy poly relationship lol