Screen brightness decreases when I middle-click
What you need to do is change the key mappings on your mouse. What this does is tells your computer what to do when you press key X.

For Mac… 


  MapKeyboard is a free program to remap your keyboard. Using MapKeyboard, you can alter any key on your keyboard to function as a different one or altogether disable it as well. This is an essential tool for laptop users and other people with keyboards whose keys differ from the standard 101 keyboard layout.


For Windows…


  SetPoint Software
  
  Logitech SetPoint™ software lets you customize your mouse buttons, keyboard F-keys, and hot keys. Use SetPoint to control tracking speeds and configure other device-specific settings. It can also notify you of our device’s battery status, and whether Caps Lock and Num Lock are on.