Here is the step by step:

1. Introduce yourself to Zuri and Joey when you're able to go to the Mall for the first time
2. Progress with the Main Storyline and talk to Joey every now and then until he ask for your help with his prank
3. Accept Joey's request and go talk to Zuri, tell her that there is somebody that needs help in the Bathroom. 
4. Go to the Bathroom and you'll get the threesome scene with Joey and Zuri ( Unrepeatable )