In a large mixing bowl, combine the softened cream cheese, sour cream, and mayonnaise. Mix until smooth and well blended.
Stir in the grated Parmesan cheese, shredded mozzarella cheese, chopped artichoke hearts, and thawed spinach.
Add the minced garlic, onion powder, salt, black pepper, and crushed red pepper flakes (if using). Mix until all ingredients are evenly combined.
Transfer the mixture to a slow cooker and spread it evenly. Cover and cook on low for 2 to 3 hours, or until the dip is hot and bubbly.
Stir the dip before serving and adjust seasoning if necessary. Serve warm with tortilla chips, bread, or vegetable sticks.