Check out Johnny Soporno's material. He has in-depth stuff about maintaining MLTRs.
As long as you're being honest about the fact that you're seeing other girls from the outset (but no need to go into specifics) you're basically ok. If they stay with you, they're consenting to that choice. It's a double edged sword because YOU cannot then be jealous if they're seeing other guys (again, neither of you needs to actually talk about who else you're seeing, just accept the general fact that it's happening).
When it comes to having a party and a bunch of them are there, all the girls you know will kind of assume you're fucking everyone, so there'll be a lot of sexual tension and jealousy going on, but if you're above it and have a great time, then they'll basically vie for your attention and you can choose who to fuck, or even get a threesome going
