15-5 with one win in the BTT is such a solid 3 seed that it's actually a LOCK, full stop.
However, while a 72-71 win in the quarterfinals after a 15-5 regular season would be a 3 seed LOCK, a 72-71 loss would not only remove our erstwhile LOCK status but actually be so damaging that we would suddenly be a 4 seed (presumably; maybe you think the BTT is so important that we would be a 5 seed?)
Literally one basket is enough to swing us from 3 seed LOCK to NOT 3 SEED.