Okay, so for having sex in public, it's pretty easy. Just do a bunch of things outside (if you've already dealt with Charles, you can make faster progress obviously since he won't interrupt you) and keep Emma around for as long as you can while amassing as large of a crowd as possible. If you continue with this for a while, you should get a message that says that she's become something of an exhibitionist and at that point, whenever you have sexy times with Emma outside and you get a crowd, she'll say something along the lines of the two of you teaching them how it's done.
For doing a threesome, I believe it has to do with love and inhibition. I think if you get their love and inhibition high enough and you are the Master or higher of one of the girls, then you can start fooling around with any other girl that you have a high enough stats with to fool around with around which ever other girl you want a threesome with, you should get a scene where you get pulled over and you're basically asked if it's okay to have a threesome relationship with whichever other girl you've chosen. However, I'm not sure if it's Master-status, higher or lower. It just kinda happened to me with Kitty and Emma, so you may just have to do some experimentation or just cheat max status for all of the girls.
Hope this helps~!
