The power supply should come with 3 separate pcie 8pin to 2x6+2 cables. I would prefer powering 3080ti with 3 separate cables but you could probably get away with using cable that fans out to 2x 8.
If you were to buy cable (hopefully you don't and 3rd cable is just misplaced somewhere, that's if you bought the psu new and not second hand with missing cables) then directly from Corsair.
Never mix modular cables from different psu brands (and different models between brands), also goes for singular bought cables, unless you're 100% sure and tested the pinouts are the same for your psu because they often aren't and could be feeding power to one of the pins of the device (ie gpu or drive) when it shouldn't be and cause major damage...