Many laptops lack a middle button, especially those with a trackpad, and you need special software to emulate it.
Mac laptops have only one button. Right-click is pretty easy (two finger click) and not uncommon in Mac games, and the two finger drag to scroll isn't bad, but only in slower paced games. However, there is no concept of a middle click in the Mac Trackpad system settings. You need special software.
You might say "who cares about laptops, nobody games on a laptop". The Steam Hardware & Software Survey is a wealth of information about what people actually use. 70% of the Mac users on Steam are using a laptop. PC laptop gamers are harder to track, but you can get an idea by looking at the video cards and noting the mobile ones (GeForce blah blahM, Intel HD Graphics, "Mobility", and "Mobile"). By my reckoning that's 30% of the market.
For example, I have a custom built desktop PC at home for gaming with a five button Logitech gaming mouse. Tomorrow I will get on a seven hour flight. I will have my Macbook Pro laptop and 200+ Steam and GOG games to choose from. I will not have room on the tray for a mouse. The games I will be playing will be ones which work well with a trackpad.
Absolutely allow all of your controls to be reconfigured. Even if people have the buttons, there will be people who will not like your default controls. It's such a small thing to avoid your customers getting frustrated.
FWIW I generally avoid the middle mouse button because it can trigger the scroll wheel. I also avoid the scroll wheel to switch weapons because I find it slow in games with more than two choices. For something like a melee attack or crouch, I typically use F or a thumb side button.