Butt shrinking 3 months post bbl.

Thank you for your question. The pressure caused by sitting is not sufficient to affect viability of the fat cells. You are likely seeing changes as a result of decreasing swelling.  After surgery there is a predictable swelling process that mostly resolves over two months, when the final result really settles in.  

Sitting or applying pressure to the buttocks 3 months out from surgery will not affect your permanent result. It is normal for there to be volume loss ~30-50% of the volume infected over the first 3-6 months after your surgery. This will happen whether you sit on it or not.