I've also got an issue with my fan 3.
 
https://forums.evga.com/Slow-spinning-3rd-fan-on-3080-ftw3-ultra-m3209668.aspx
 
IMO, this is how the card behaves. If you go to HW info, you will see fan 1 and 2 reported. But there is no info for fan 3.

I think this is a fan control issue related to drivers that hasn't been fixed yet, same as the temperature spike readout error on iCX sensors that is NVidia driver related and has been present since the 2nd release of drivers for the 30-series cards.
 
If your HWInfo64 is not reading out all 3 fans on a 3-fan card, there should be an HWInfo update that fixes that, as my 3080 FTW3 Ultra I used to have did not have a reporting issue after an update that HWInfo put out like 2 months ago, so make sure your HWInfo is the newest version.
The fan control is something where you can always try setting the 3rd fan higher if it's running slower or whatnot, highly recommend doing a custom curve regardless, don't let the fans auto-control themselves, it's not ideal for cooling.  Set it up so that the fans are off at idle/desktop temps, but once you hit "gaming" temperatures, it ramps up - figure out what speed is a maximum acceptable "noise level" for your personal preference, and set it to max out at that speed starting at ~55C, and you should have an effectively cooled card from there on out.

My HwInfo does show the fan but it's in its own category and called right fan, not fan 3 for some reason.
 
I do have a custom fan curve enabled as you can see from my previous screenshots. The flat one was just for testing. Cooling wise everything is good with the card. The third fan is not acting idly different either but I find it concerning that 2 fans have the exact same speed% graph while the 3rd, while following the same curve, is quite different.
 
My main worry is that new sound I have been hearing for the past week or so, it's not the normal sound of the fans ramping up and down. I am honestly not bothered even if the fans are loud since I've got headphones on most of the time, I am bothered by the fact that this new weird sound came up recently combined with the fan not being as stable as the other two. Those two things lead me to believe the fan is faulty in some way, such as a bad bearing or not properly lubricated leading to that weird sound. I just don't really know much about this stuff so I'm trying to find out if some people had similar issues to inform myself. 
Lastly, this sound is only noticeable when the fan speed is over 60ish%. It only come from the 3rd fan as turning it down on it's own the sound goes away.
 
Worst come to worst I will have to RMA the card but I'd rather not have to go through this entire process unless absolutely necessary.
 

I also have problems with fan3 on my 3080 ftw3. Extremely annoying modulating sound. Sounds like it's pulsing up and down in RPM.

Ekamin
Deinstall msi afterburner if u have it installed. It fixed it for me because it overrides evgas fan curve which is 0 rpm or >40 so the sound cant appear anymore.

It's enough just to set more %% in MS AB. For example, I set 50% and have 860 RPM on all 3 fans (bios w/o RBar, I switched to that as RBar vbios isn't good at fans control) without start-stops.