Hi Sylvain,
Since the PCB of the Asus 3090 strix is nearly the same as the Asus ROG Strix RTX 3080Ti OC edition, the backplate water block BP-VG3090ASTEN should be compatible. But BP-VG3090ASTEN has to be installed with the front water block BP-VG3090AST. Do you already have the front water block?