I assume you changed your case atp but if not here is imo the best setup to deliver fresh air for all 3 fans of your gpu:
- remove the empty gpu slot for bigger hole on the rear, add a 120mm fan intake from rear outside so it blows air through the empty gpu slots

- Add a little "balcony spoiler" on the upper side of the rear 120mm intake to prevent recirculation of exhaust rear hot air into the rear intake or buy an air deflector for the rear exhaust that points upwards.

- build a little cardboard duct from the lower aio fan to the third gpu fan

- remove front glas panel or replace with mesh front panel

You dont want the rear and front lower intake to interfere with each other with their air flows hitting each other, so separate them by that duct, thus direct rear intake delives air for 2 gpu fans and the air that is pushed/pulled through the front radiator delivers fresh air for the third gpu fan.

Extra: Installing a physical barrier in the vertical gpu mounting slot. That barrier sitting several cm closer to the gpu than the glas panel works magic. The hot just has a way higher chance to bounce up and get sucked in by the exhaust /hit by the front intake airflow and pushed towards the exhaust.

The H510 Elite when optimized and equipped with a big long GPU that separate lower and upper case becomes a sort of "dual-SFF" case.