I just got a 3rd monitor for my setup and after plugging it in one of my monitors wouldn't display anything.(I have 2 27 inch LG monitors that are the same and a 55 inch tv that i sometimes use and one of the LG's wouldn't work) After a while of trying to figure out what was wrong with my monitors I found out after going through the NVIDIA Control Panel that I could either use my 2 LG monitors alone or if I wanted to use more than 2 displays, I couldn't use both LG monitors, I could only use one or the other. Not sure what's going on but I figured someone here might know. Here's two screen shots of what I see. (The one not in use is display 2)
What model is the GPU? If you go to Configure Surround at the top can you setup three of the monitors in 3D Surround and one as an auxiliary monitor? Can you set the monitor that's disabled from disconnected to extend in the Windows Display Settings instead of Nvidia Control Panel?
What you are likely seeing is DSC (Display Stream Compression) limitations for your pixel bandwidth. The RTX 3090 Ti can support a maximum resolution of 7680x4320 when using a single DP 1.4a/HDMI 2.1 cable with DSC at 12-bit color and 60hz. This means if you cut this in half, you can support two 4k 60hz displays under the same conditions. To add a third display, you'll need to either compromise on refresh rates (30hz) or resolution (dropping down to 1440p or 1080p).
You can do some shenanigans like adjusting the color depth, disabling HDR and G-Sync to save some bandwidth, but there is no guarantee it will work due to how much bandwidth you are needing. I would recommend starting with a lower resolution on each panel then slowly increasing until you hit your limit. From there, tinker with Nvidia's custom resolution feature to define specific refresh rates for each panel until you can get them all to display within the bandwidth limitation.
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