page 64 of rev 5 of the ref manual says that the FSMC is at 0xa0000000 and it's on bus AHB3, but it doesn't say the AHB3 base is at 0xa0000000.  The datasheet itself, figure 18, the memory map,  shows that AHB3 is 0x6000000-0xA000 0FFF

FSMC just happens to be the only peripheral on AHB3 :)