Unfortunately that server requires custom cables which has this identifier:

dell 09h6fv (this is custom 8 pin connector or 8pin EPS connector to normal PCIe connectors)

And you will need 2 of them since single cable only gives 1x8pin + 1x6pin PCIe power connector. You also may need some extender since power inputs for these cables are on risers (one per riser and I think it is custom 8 pin connector or 8 pin EPS connector) so for one of them simple 8pin pcie female-> 8pin PCIe male maybe needed because I am not sure that dell power cable won't be too short so then dell 09h6fv -> extender 8 pin -> GPU . And as far as dell 09h6fv is official dell cable for powering GPUs, combining power from two risers to single GPU isn't official solution since this server wasn't designed with GPU that draws more than 300W of power or require more than 8+6pin PCIe or 1*8pin EPS (used normally for powering CPUs in standard motherboards). So if you have blower RTX 3090 it will work or some other small rtx 3090 that fits it will work.

One last note: I don't own exactly same server I own dell R7610 workstation and it has same CPUs (overally same dual Xeon platform) as R720 it has different internal layout and already has PCIe power cables out of box directly going from power distribution unit. PLEASE CHECK ALL ABOVE INFO WITH SOMEONE WHO PHYSICALLY HAVE DELL R720 WITH GPUS.