Have you tried reinstalling the whole game from scratch? Your issues are not something that default settings should bring (i.e. they don't use mouse3-5 for anything)
Which mouse do you use and does it have any software installed on your computer? You can bind mouse buttons using external software as well so it can also be related to that. For example mouse3 could be bound to ctrl, so the game would understand any mouse3 stroke as ctrl. This can also be achieved using a variety of different programs, not necessarily related to your mouse. I would check that.
Does the problem only happen with mousebuttons 3-5? Does the same thing happen if you bind weaponbank4 to mouse1 and press it together with WSAD or however the problem occurs?
You can also try binding a different key to the scores-popup and see if the problem still exists.
Also instead of GUI, you could try placing your binds in a config file to see if it makes a difference.
Here's what you config should look like (can just be these 3 lines):
bind mouse3 "weaponbank 4"
bind mouse4 "weaponbank 5"
bind mouse5 "weaponbank 6"
You also could try typing /unbindall in console and then resetting your controls back to default. Should take care of any funny binds you might have.
E: Added some comments.