Cook the Pasta:
Add macaroni, water, salt, and butter to the Instant Pot. Stir to combine.
Close the lid and set the valve to "Sealing." Cook on high pressure for 4 minutes.
Quick Release:
Once the cooking time is up, perform a quick release of pressure. Carefully open the lid.
Prepare the Sauce:
Stir the pasta to loosen it up. Add milk, cheddar cheese, mozzarella cheese, Parmesan cheese, black pepper, and garlic powder (if using). Stir until the cheese is fully melted and the sauce is creamy.
Adjust Consistency:
If the sauce is too thick, add more milk a little at a time until the desired consistency is reached.
Serve:
Serve immediately while warm and creamy.