Preheat oven to 350°F (175°C).
Place pretzels in a ziplock bag and crush with a rolling pin into small pieces (not fine crumbs).
In a medium bowl, combine crushed pretzels, 3 tablespoons sugar, and melted butter.
Press the pretzel mixture firmly into the bottom of a 9x13 inch glass baking dish.
Bake the crust for 10 minutes, then allow it to cool completely.
For the creamy layer, beat softened cream cheese and 1 cup sugar until smooth and fluffy.
Fold in the whipped topping with a gentle hand until fully incorporated.
Spread this creamy mixture evenly over the cooled pretzel crust, being careful to create a solid layer that completely covers the pretzels and seals them from the topping.
Refrigerate while preparing the pineapple layer.
For the pineapple layer, pour undrained crushed pineapple into a saucepan and bring to a boil.
Add the gelatin and lemon juice (if using) to the hot pineapple, stirring until completely dissolved. Continue boiling for 2-3 minutes, stirring occasionally.
Allow the pineapple mixture to cool to room temperature – but not to set.