Does anyone know if it's possible to drive three HD/QHD monitors via a single thunderbolt port on an M1 Mac, and if any docks exist that would support this use case?
Many of these docks and Apple support documents say they support dual displays at 4K resolutions, but it's difficult-to-impossible to find information on # of displays supported at lower resolutions.
The short answer is no, unfortunately. There is a limit of two monitors per Thunderbolt port. My understanding is that this limitation lives somewhere at the DisplayPort spec level, but I don't know the specifics.
The longer answer is that you might be able to, depending on resolution/refresh. This will need both hardware and software for "DisplayLink". I've never kicked the tires on this setup, but I believe it's basically setting up a single frame buffer that will span two physical devices.
IMO it isn't very viable, especially once you're past 60hz HD resolutions, and also because the macOS drivers/software sometimes lag behind major OS releases or even point releases.
I hang a 4k@144hz and two 1440p@144hz screens off an M1 Max. It's basically one monitor per cable at that point (docking is two Thunderbolt + HDMI). One TB cable goes to a CalDigit TS3+ for power, USB accessories, and one of the 1440p monitors. The other goes straight to the 4k monitor.
One question to clarify your setup -- So the 4k monitor is connected directly to your mac with a thunderbolt cable, one of the QHD monitors is connected directly to your mac through HDMI, and the second QHD monitor is connected through the CalDigit dock?
If you don't mind, can I ask why not just connect both QHD monitors through the CalDigit dock?
Many of these docks and Apple support documents say they support dual displays at 4K resolutions, but it's difficult-to-impossible to find information on # of displays supported at lower resolutions.