Most mouse have a ''3th'' button by pressing the scroll wheel.
However if you want to rebind mouse buttons beond the standard configuration, you often need to install your mouse specific software. Otherwise windows might not recognize other buttons beond 1L-2R and 3scroll.
This differs per mouse however, check your devices to know for sure if windows has your mouse selected properly and not a default mouse. (if it does not, select your mice as standard in device manager or install its software)

If that does not work, it is a problem on Valheims end, and i am afraid only the developers can add changes that support button imput changes.