Grate cheese keeping the medium cheddar separate from the sharp cheddar and the pepper jack. Set the medium cheddar aside.
Cook macaroni. Pour noodles into a medium to large size pot. Cover with water. Bring to a boil and cook until soft--about five minutes from the time the water boils. Strain and set aside.
Pre-heat your oven to 375 degrees.
Make a roux. In a Dutch oven, melt the butter over medium-low heat. When fully melted, add the flour and stir until it is fully incorporated into a paste.
Add the milk and stir until the roux dissolves into the milk. Continue to stir until the mixture begins to thicken.
Add the salt and garlic powder.
Add the sharp cheddar and the pepper jack to the pot one handful at a time stirring constantly. Let the cheese completely melt before adding another handful. Repeat until all the cheese is melted into the sauce.
Remove from the heat and add the sour cream. Stir to incorporate.
Top with the medium cheddar cheese. Sprinkle pepper uniformly on top.
Cover and bake at 375 for 30 minutes. Remove lid and broil until the top browns. Usually around 4 minutes but all broilers are different so check frequently.
Let stand until it stops bubbling. Serve in the Dutch oven.