I'm going to add a custom cooling solution to my EVGA RTX3090 FTW3 Ultra with the hybrid cooler kit. The card is still sitting in its box, but I plan to upgrade it with the proper "hybrid" firmware for correct fan control for the shroud + radiator fans through Precision X1.
 
I will be adding some additional fans to help cool the backplate and the VRAM chips, basically, I plan on using 3 Scythe 92mm slim fans mounted in a bracket that can be fastened in the PCI slots, so the 3 fans will sit directly above the RTX3090. I'll experiment with fan direction, and possibly adding some small heatsinks to the backplate, in order to see what works best.
 
Anyhow, my concern is fan control. The 3 fans are PWM fans, and I'll use a 3-1 splitter so all the fans can be controlled by the same PWM signal, I want them to run at the same speed anyhow. Ideally, I would like to connect this to the EXT FAN header on my RTX3090 FTW3 Ultra, but I can't find any information re this fan header or if this is possible at all. Ideally, this fan(s) should react to the VRAM temperature, and not the GPU temperature, as the VRAM temperature can be very high even when the GPU temperature is relatively low. Does anyone know if this is possible?
 
And also, where to find some info re the usage of the EXT fan header on this card? Eg what can this be used for and how to control it?
 
I have to say it's a bit disappointing to buy a graphics card this expensive and then find that it has features that are not even documented. Or at least not in a way that can be easily found.