Squeeze the thawed spinach in a clean kitchen towel to remove all excess moisture.
In a large bowl, mix together cream cheese, sour cream, and mayonnaise until smooth.
Add the drained spinach, chopped artichokes, mozzarella, Parmesan, garlic, salt, and pepper to the bowl and stir until well combined.
Transfer the mixture to a greased slow cooker and spread evenly.
Cover and cook on low for 2 hours, stirring occasionally, until hot and bubbly.
Serve warm with crackers, baguette slices, or tortilla chips.