Mix pretzels, 3 tablespoons sugar and margarine. Press in bottom of a 9 x 13 dish. Bake 8 minutes at 400 degrees. Mix cream cheese, sugar and cool whip. Layer on top of cool pretzel crust. Make jell-o according to package directions and refrigerate until set. Mix berries and pecans into jell-o. Place on top of cream cheese mixture.
NOTE: You can also use confectioner's sugar. In fact, I think it works better than regular sugar.