In a medium bowl, mix cream cheese, drained spinach (make sure to squeeze out excess water), mozzarella, and feta until well blended, using a silicone spatula.
Brush melted butter on the bottom of a 13×9 baking dish or sheet with sides at least an inch high. Place a sheet of phyllo dough and brush it with melted butter.
Repeat steps 2 and 3 until half of the phyllo dough layers are used.
Spread all of the spinach pie filling over the prepared phyllo dough, ensuring an even layer.
Continue layering phyllo dough and melted butter, alternating between the two. Cut the pie into pieces.
Whisk together milk, eggs, salt, and pepper. Pour this mixture over the spinach pie, ensuring all pieces are assembled in the baking dish. Let it sit for 15-20 minutes to absorb the mixture.
Bake in the oven at 375 degrees for 30-35 minutes, aiming for a golden brown crust. Allow it to cool for about 10 minutes before serving warm. Enjoy your savory phyllo pie!