If you sit on your butt for the first 3 days after a BBL is enough time to kill the fat cells??

I don't believe you have ruined your butt. Even though I recommend minimal sitting for the first 6 weeks, many patients have to travel home in a seated position. It is ideal to avoid trauma to the area, as blood vessels have to grow into the transferred cells. I would recommend that you minimize sitting for the first few weeks. Please consult with your surgeon. 

It may have but you won't see the affects for a couple of weeks. Please avoid doing that as advised by your surgeon, and follow up with them so they can monitor your results and recovery.

Ideally you do not want to sit on your buttocks for any length of time during the early post-op period. I usually allow people to sit by about two weeks and then moving around regularly as well. 

To my knowledge there is no published study documenting the effects of pressure on the buttocks after fat transfer (BBL). However, since the fat is a graft and will ultimately die if it does not receive adequate blood supply many surgeons recommend avoiding pressure on the buttocks after surgery for a variable amount of time. I personally recommend strict avoidance of pressure for the first week except when absolutely unavoidable. The second week many patient have to return to work and although they may sit, I encourage them to avoid slouching back on the upper part of the buttocks where most of the fat has typically been transferred. I have experience with performing fat grafting to the buttocks on tummy tuck patients and have seen that a good part of the fat survives the pressure of lying on the buttocks. So in your case although there may have been enough pressure to possibly affect the 'take' of the fat but I would expect a significant amount will survive. Best of Luck  Dr Harrell