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.

Keep in mind that "Mouse-3" and "Mouse3" are differernt button codes in Unity ("Mouse-3" is the scroll wheel button and "Mouse3" is the first side button after LMB and RMB).

Originally posted by avatar.zero:

Keep in mind that "Mouse-3" and "Mouse3" are differernt button codes in Unity ("Mouse-3" is the scroll wheel button and "Mouse3" is the first side button after LMB and RMB).

thnx didnt know :-) but use programmable 9btn mouse will try in TLIML unity game

you installed your mouse properly with the right drivers so your comp and game can detect the buttons properly?