Preheat the Oven: Start by preheating your oven to 375°F (190°C) and greasing a muffin tin with cooking spray.
Whisk Eggs: In a large bowl, whisk together eggs, milk, salt, and pepper until well combined.
Add Cheese: Stir in shredded cheddar, mozzarella, and grated Parmesan cheese into the egg mixture.
Fill Muffin Tin: Pour the egg and cheese mixture evenly into the greased muffin tin, filling each cup about three-quarters full.
Add Vegetables: If desired, add diced bell peppers, spinach, tomatoes, or any other veggies of your choice to each muffin cup.
Bake: Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the frittata cups are set and lightly golden on top.
Serve: Remove the cheese frittata cups from the oven and let them cool for a few minutes before carefully removing them from the muffin tin. Serve warm and enjoy!