Do you use some vendor-specific application to control the fans in Windows?.

And I find it pretty unlikely that they would what, use the defined interface for fans 1 and 2 but use some crazy one for fan 3? And even if that were the case, it wouldn’t prevent Nvidia from adding support to the driver, AMD GPUs are able to detect all three fans no matter who makes it, and AMD hasn’t added anything vendor-specific to their drivers. I mean, if that were the case, fans 1 and 2 wouldn’t be individually controllable either.

third fan not defind in reference design so vendors use a different interface.

And since they don’t use hwmon, we can’t look and see if Linux itself is detecting it (which it is though, because sometimes randomly it will work, but VERY rarely, and I can never force it). Nvidia has never made a card themselves with more than two fans to my knowledge, and there is so much old legacy crap in the Linux (and probably Windows but idk) driver that it’s REALLY easy to believe that they just never even added an interface for a third driver. I think this is the most obvious solution. Both fans 1 and 2 are reported as targeting “GPU, Memory, and Power Supply.” So obviously the third one would be the same, or Nvidia-settings isn’t actually reading what they’re actually supposed to be targeting, either way it doesn’t imply anything about more than two fans. I don’t see how that would imply anything about detecting more than 3 fans. It should be there, at least the description for nvidia-setting’s gpufantarget implies: I’m happy to provide any information that might be needed, but I would imagine it’s a simple process to just add capabilities for a third fan to the drivers. I pay 1700 USD for a top of the line GPU and I can’t even have drivers that report all the fans. In Linux, when I set manual fan control, I can only control the first two fans because there are only two fans reported. This is the second RTX 3090 I’ve had this issue with, and It works perfectly fine in Windows, I can go in and change the sliders for all three fans, and all three fans respond accordingly.

Well, I can’t get my third fan to come on. This makes sense for FE/Reference cards that only have two fans, but most AIB cards, especially newer/high end ones, have three fans and are not designed to run with only 2.

So, the Nvidia drivers on Linux only report up to two fans, regardless of how many fans the GPU has.

Linux gpu fan control