Responses
Maybe second chapter of "Incest"
I'll keep looking, but so far I found this one: http://www.mangago.zone/read-manga/nibiiro_no_hana_gou/
I haven't read it, but the synopsis sounds kinda close to what you're describing. Hope that's it!
It's smut. I think a one shot. And its about 3 brothers: the two oldest sort of gangbang the youngest one (only them 2) and when the youngest one knock out the oldest sex the middle one and there are flashbacks about their past and something to do with their dad...? It's not a "sane" manga, sort of dark-ish, not too bad