Superawsomemans, There are Sims 3 mods for nudity, attaching genitals, etc., but until recently (March) none for sex. If you have played Sims 3 you will have seen that the actual sex act is never displayed; it always takes place under covers, under the water, or in a rabbit hole. You can, through a tedious posing process, create static sex scenes, even threesomes (or more). Now, there is a mod for creating video of visible sexual interactions between two characters. It is available on Sexy Sims 2 here:
http://www.sexysims2.com/showthread.php?t=173861.
You will need to join Sexy Sims 2 to see it. Joining is free, and if you want to do with your sims what it it sounds like, joining will be to your benefit to get all the mods. Even though the site is called Sexy Sims 2, it has Sims 3 mods too.
I haven't tried the sex mod, so can't vouch for it. Since this mod is limited to two sims, it looks like at this time to get the threesomes you want you will only be able to do it by creating static poses.