In a large bowl, combine the softened cream cheese, sour cream, and mayonnaise. Mix until smooth.
Stir in the Parmesan cheese, mozzarella cheese, and Monterey Jack cheese.
Add the chopped artichoke hearts and squeezed spinach to the cheese mixture.
Mix in the minced garlic, salt, and pepper.
Spread the mixture evenly into a baking dish (an 8×8 inch dish or a pie plate works well).
Bake for 20-25 minutes, or until the dip is hot and bubbly and the cheese is melted and lightly golden.
Serve immediately with your favorite dippers like tortilla chips, crackers, or bread.
Notes
For a slow cooker version, combine all ingredients in a slow cooker and cook on low for 2-3 hours, stirring occasionally, until hot and bubbly.
You can prepare the dip up to 24 hours in advance. Cover and refrigerate. Bake as directed, adding a few extra minutes to the baking time if starting from cold.
To reheat leftover dip, cover and bake in a 350°F (175°C) oven until heated through, or microwave in short intervals, stirring between each.
Consider using a mix of cheeses like Gruyere or Provolone for a different flavor profile.