Preheat the oven to 350℉.
In a large bowl, mix the ricotta cheese, mozzarella cheese, cream cheese, spinach, eggs, salt, and black pepper. Stir together until well blended.
Place about 1½ cups of marinara sauce in a 9×13-inch pan. Spread it around to cover the bottom.
Place the mixed filling into a plastic bag. You can use either a frosting piping bag or a gallon zip bag. Snip the corner, gather the top and squeeze the filling into the corner.
Hold an uncooked manicotti shell in your hand and squeeze the filling inside the shell. Let it overflow on both sides.
Place the filled shell into the sauce-lined pan. Repeat with remaining manicotti and fill the pan with the stuffed shells until you run out of shells and filling.
Cover the pasta completely with marinara sauce, using about 2½ cups. Sprinkle Parmesan cheese on top.
Bake, covered with foil, for 50 minutes. Remove foil and bake an additional 10 minutes.
