In A Small Bowl, Beat The Cream Cheese, Mayonnaise And Sour Cream. Stir In The Artichokes, Spinach, Mozzarella Cheese, 3 Tablespoons Parmesan Cheese And Garlic Salt. Remove Stems From Mushrooms (Discard Stems Or Save For Another Use). Fill Each Mushroom Cap With About 1 Tablespoon Of Filling. Sprinkle With Remaining Parmesan Cheese. Place On Foil-Lined Baking Sheets. Bake At 400