I have had three cats at a time, and I remember when we introduced the 3rd (a male) to the 2 females who were good friends, they did not welcome him at all. They eventually lived peaceably, but the one absolute rule that they followed was that they would not share a litter pan with him. It's important to provide an extra litter pan, or maybe 3, as far apart as possible, so that the girl kitty can avoid where the boys go. I don't know why, but they seem to have rules about this and are not interested in our opinion.
Those 3 cats lived together for 13 years. The two females remained friends but always kept their distance from the male. All of my cats were altered - always.
I currently have 2 females and after a couple of days of mild resentment the older kitty started grooming the kitten. They use the same litter pan with no complaint. The younger cat keeps the older cat's whiskers trimmed short, which drives me nuts. I think that's what's going on in my avatar picture.