The founders edition card seems to be precisely 3 slots, but you will need to be quick aqcuiring one, since I hear rumors of really low stock due to a $150 cooling solution. Don’t let the GPU sit on the PSU shroud you will strangle it for airflow.
That said, perhaps a 2.5" slot AIB version like the Asus TUF RTX3090 GAMING would be a better version? At 52mm width, that means roughly 10mm to spare which should be enough for airflow. I have also seen some other AIB models with 2.5" height floating around, too.