Big Sur Clamshell mode
 

I was running into the issue of not being able to run clamshell mode in Big Sur. I seemed to have fixed it by running:

sudo pmset -a disablesleep 1

and entering my password.


To stop sleep entirely:
sudo pmset -a disablesleep 1

To revert, allowing sleep again:
sudo pmset -a disablesleep 0

Hope this helps some people

Another method
 

Another way to have clamshell mode work is to have 1 display cable go directly to the Mac (or some dongle), and then the other display cables connect to the DisplayLink hub.

For example, I use a TB3 cable to connect directly to a display via the Mac TB3 port, and then 2 DP cables to connect to the other two displays via the hub.