Most laptops with discrete Nvidia and AMD GPUs also have onboard/integrated graphics and only use the Nvidia/AMD GPU when something graphically-intensive is happening (playing a game, video editing or encoding/decoding, etc). They call this "hybrid graphics".

However, the HDMI port on the laptop (as well as the USB-C graphics) is wired directly to the Nvidia GPU (I'll call this the "dGPU" from now on). This means that when an external monitor is plugged in but nothing graphically intense is being done, the screen is rendered on the iGPU, then sent to the dGPU to send over the HDMI port.

The hand-off between the dGPU and iGPU (called "reverse PRIME") is basically voodoo magic. People have tried to get it working in Linux, but there's a bunch of issues with it.

To get dual monitors working properly on my work laptop (Lenovo X1 Extreme Gen5 with an RTX3050), I have to go into the BIOS and force it to only use the dGPU (disable the hybrid mode). If I don't do that, the external monitor renders at maybe 5fps? A coworker got it working by instead forcing the Nvidia card to always use a high clock speed for the RAM instead of reducing it to save power, but I haven't tired that.

This is a laptop-specific problem, only for laptops with hybrid graphics. I have no problems using three monitors on a desktop PC.