Lightly coat the inside of a 6-quart slow cooker with cooking spray. Stir together evaporated milk, milk, and water in its bowl. Sprinkle flour on top and whisk constantly until smooth. Stir in mustard, salt, pepper, 2 cups cheddar, Monterey jack, and Parmesan. Fold in macaroni and broccoli, making sure the macaroni is immersed in liquid.