Preheat oven to 400 degrees F.
For the crust, mix the pretzels, butter, and 3 tablespoons of sugar. Press this mixture into a 9 by 13-inch pan and bake for 7 minutes. Set aside and allow to cool.
In a mixing bowl, beat together the cream cheese and 3/4 cup of sugar. Fold in the whipped topping, and spread over the cooled crust. Refrigerate until well chilled.
In a small bowl, dissolve the gelatin in the boiling water, and allow to cool slightly. Add the strawberries and pineapple, and pour over the cream cheese mixture. Refrigerate until serving time.
To serve, cut slices and serve with a dollop of whipped topping.