Preheat oven to 400 degrees. Spray a 13×9 pan with non-stick cooking spray and set aside.
Cook the shells for 9 minutes and then drain and let cool.
Mix the ricotta, mascarpone, 1/2 of the Parmesan, 1/2 of the mozzarella, and the parsley (or you can wait and put parsley on top). [I use about 1/2 cup of fresh parsley. I put a little salt and pepper in with this mixture.]
Mix the marinara and 3 cheese sauce together.
Put half the sauce mixture on the bottom of the pan. Stuff the shells with the cheese mixture so they are full but still close. Put them face up in the pan.
Spread the rest of the sauce mixture on top of the shells. Sprinkle the rest of the Parmesan and mozzarella on top of the shells.Sprinkle Italian seasoning on the top and the parsley if not mixed in with cheese.
Cover and cook for about 30 - 40 minutes at 400 degrees.