I prefer the girls in harem not playing with each others unless the MC want them to, to me it's better if they focus on the MC, since they are part of his harem, that means they want him, not each others. If they start focusing each others during threesomes instead of the MC, then later they can fuck around without the MC needing to be there, so that makes it not an harem but a polyrelationship where everyone like everyone. But I guess if girls playing between each others during threesomes are what you want, there's nothing more to say, to me lesbians in harems tend to kill the fantasy of an harem, besides in normal harems, their members don't need to be lesbian or bisex to be a part of the harem, they can be straight girls wanting to share a guy, but it's your game, so good luck with it.