Prep. Heat oven to 375F. Line a baking sheet with foil or parchment.
Make the bread bowl. Slice the top off the sourdough loaf and hollow out the center, leaving about a 1-inch border around the sides and bottom. Cut the removed bread into chunks for dipping.
Toast the bread bowl. Place the bread bowl and bread chunks on the baking sheet. Bake 8 minutes to lightly toast. Remove from oven.
Mix the dip. In a bowl, stir together cream cheese, sour cream, and mayo until smooth. Add mozzarella, cheddar, Parmesan, garlic, onion powder, paprika, pepper flakes, and a pinch of salt and pepper. Stir in any optional add-ins.
Fill and bake. Spoon dip into the bread bowl. Bake 18 to 22 minutes until bubbly and hot. For extra golden top, broil 1 to 2 minutes at the end.
Serve. Serve immediately with toasted bread chunks, chips, crackers, or veggies.