Yeah these 3090 with their 24gb (lol) vram are a complete nightmare to cool, the 3080's as well to some extent but not as bad as the 3090.
My way of thinking for my 4x 3090 is fully water-cooled or don't bother.. 3080's are a bit better on air but not massively.
All you have to do is replace thermal pads with gelid extreme... fixed my memory heat problems on 3090.