How do I reassign mouse button 3?

See I did that, but as you can see when I set it to mouse button 3, I get a yield sign. I assume that is because that key is the hotkey for this "currently open applications" thingy that appears when I click the middle button. I just have no idea where I unset the middle button so that I can set it here.

When I do that, I still get an a yield sign but it the middle click does do the application windows and overrides the "open applications" thingy despite it having the yield sign still. The secondary mouse button simply right clicks as it does normally and does not function as mission control.

The application windows here looks lke this:

It simply separates out the application I have on top. I'm not sure what purpose this actually serves... but that is what it does.

The "open applications" thing I am reffering to that was appearing before looks like:

And so you can see:

There is no middle mouse button option, though I have confirmed that mouse button three is the middle mouse button.

I see nothing helpful here.

Or under options: