There comes a point you will need to go to the clinic to talk the the Nurse on the 2nd floor, who will talk about a drug used to increase chance of pregnancy.
1. Talk to the nurse on 2nd floor of clinic.
2. Talk to the front desk lady 1st floor about going to 3rd floor.
3. Enter evelator (help the old lady fix her camera).
4. You're on 3rd floor, walk in the room, take the drug on the table --> confronted by the scientist Priya.
5. Talk to Priya --> she will give you the drug.
6. Take the drug right before breeding with Diane. --> keep doing it until she texts you about her being pregnant.
7. Wait (for weeks in my case, you will see Diane's belly getting bigger gradually).
8. Eventually, receive a text about the child being born --> go to clinic.
9. Wait some more (after leaving clinic, Diane will be holding the kid all the time --> eventually she will be back at the barn without the kid and can be bred again).
10. Scene triggers when Diana is present (evening, living room / TV room) as you go in Debbie's room --> Leading up to Threesome.
11. Follow the story to the end. Now you can have threesome by entering Debbie's room in evening.
You're welcome.