Preheat oven to 350 degrees. Place pretzels in a 1 gallon size re-sealable bag and crush with a rolling pin.
CRUST: In a medium bowl, combine crushed pretzels, melted butter and 3 tablespoons sugar. Stir together until well blended. Press the pretzel mixture evenly into a 9 x 13-inch baking dish and bake for 7 minutes. Allow crust to cool completely before adding the other layers.
FILLING: In a large bowl, beat softened cream cheese and 1 cup sugar together until smooth. Fold in Cool Whip. Dollop filling over the pretzel crust then spread evenly with an off-set spatula (be sure to seal the cream cheese layer against the sides of the dish so that the Jell-O mixture will not be able to leak through when added).
TOPPING: In a separate bowl, whisk together orange Jell-O and boiling water until the Jell-O is completely dissolved. Set aside to cool slightly (about 10-15 minutes). Gently stir in mandarin oranges. Pour the Jell-O/Mandarin orange mixture over the cream filling, spread evenly.
Place in the refrigerator and allow Jell-O to set up completely. Cut into squares; serve with a dollop of whipped cream!