Out of the four vents on the dash only vent number three is blowing hot air?

So it is important to try to understand what is going on, I would put the fan speed on low with the engine warm. Then cycle the hot to warm controls to see if you can hear the blend door moving, there should be to of them, then cycle through the modes and listen as well. We are trying to see if it is blend door issue or this truck has an engine coolant flow control valve. Here is a guide to help you check the fuses which I have listed the ones you need to check in the images below.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Chances are the driver's side bled door actuator is not working which is common. Here is the location so you can swap it out and recalibrate it. Check out the images (below). Please let us know what happens.

My truck is a 2020 3500 crew cable 4wd 6.6 Duramax. Do these instructions still apply since the parameters in this thread are fit a 1500 4wd 3.0l Duramax?

One thing that is still perplexing is if the driver side blend door is failing (vents 1&2) then why is vent 3 hot but vent 4 is cold like vents 1 and 2?

*Crew Cab.

Yep, that is strange, doing more research I see that there is a cam system what works the vents. I don't know if you can get a visual on it but here is what it looks like, if you use a flashlight see if the cams are actuating the control arms which work the vents. Here is what it looks like. It is on the driver's side above the throttle pedal.