First of all you have to look porn at the start of the game. In Fact in the porn you see the star is Trina.
At the start of the party, when Bob ask you to give to Jazz the fake cocktail you have to tell Yes, but you DON'T get the cocktail to Jazz, keep it in inventory.
After you need to have that Vivian and Bob NOT have sex (if you have orgy you can't have threesome), the easy way to be sure of it is to give the heart of Viv (the 1000 point, she loves you).
Carry Jazz after the second "three hearts" (when you remember of the cocktail she love) and ask to Bob to make it for you, but NOT give Janet to him. He make the cocktail for you "free" after all.
Have the standard game till the third "three hearts" of Trina, here you have to do this things in this order:
- Talk to Trina and click on the hearts, you notice that she is a pornstar, and you can blackmail her for this thing.
- Go to talk to Bob, and click on the Trina Face, he tell you that you had to distract Jazz.
- Go to Jazz and give her the fake cocktail you have taken at the beginning of the game.
- Go to talk to Bob and click on Trina Face, now he want to have threesome. Now I don't remember if the threesome start immediately or you have to go to your room, but if don't start, you have to go to your room
In this way you can have the threesome.
After this, Bob make for you all the cocktail you want. So you can give one of this to jazz and go from 0 to 1000 whitout the need to talk to her.