Either in windows or if the device comes with software, change one of the buttons on the mouse to middle mouse button or change it in game to a different key. You can't change buttons in the in-game menu but you can before you launch the game.
I haven't remapped any keys, and I never use the middle mouse button. Number keys for towers/repairs/upgrades. "I" if I need to get into the character screen. The only time I absolutely have to use middle mouse is if I want to sell a tower, and that's rare.