I am three months post op for a BBL. When can I fully sit without a pillow?

You should discuss this with your board certified plastic surgeon. I generally have no restrictions for sitting or sleeping after 8-10 weeks.

Hello dear!!
Thanks for the question and provided information as well.
Each doctor has their own protocol It is very important that you talk to him about your concerns and goals. I recommend my patients not to sit for more than 30 mins. every two hours, for the first 2 weeks, and always with the help of a boppy pillow. If you have any concerns, I suggest you to follow up close with your plastic surgeon.
Hope this is helpful. Good luck :) 

Usually, my patients are able to sit directly on their buttocks at three months after surgery. Please ask your surgeon.

This is a question that you should ask your surgeon.  At 3 months post-BBL, my patients have no sitting restrictions.