well for threesomes the easiest short term way is a 100% being a cheerleader and doing the Saturday games with the random chance for the threesome and/or the Friday party's for a random chance of a threesome. however going long term if you want more of guaranteed chance and want to do it more than just Saturdays and Fridays you'll need the poly Inclination easiest way to get that is to start a game with it but even starting with it can be difficult cause you have to find other npcs with the trait and get high enough relations for them to agree to a poly relationship which can take awhile but after you get 2 or more partners it should be more straight forward about how to get threesomes i cant say for sure but i think it will give you the option when going for a booty call or it could be a date option (i can say for 100% on either of those 2 options)
as for meeting just for sex your either going to want booty calls or quickies you'll need higher Disinhibition for either of these options the same for both i think i dont remember the exact amount but i think its between 4-6 the easiest way to know whether you can booty call someone is by asking for their number (which you need anyway) and they'll respond with "sure but only for booty calls" (or something like that) than go in contacts in the booty call section click their name and see if you can ask them for a booty call if you cant you need more Disinhibition if you can than than its pretty straight forward from their
for quickies take someone on a booty call at least once (or have a sexual encounter in some way) than whenever you talk to them in locations where quickies are available (basically anywhere with a bathroom you can access) you'll be able to ask if they want one
tldr:
threesome: 1.poly relationship
2. dont know exactly but isnt hard to figure out
booty calls: 1.have enough Disinhibition
2.get phone number
3.text for a bootycall
quickies: 1.have sex with person
2.go to location with accessable bathroom (can be inside or outside the building)
3.ask for quickie