You will be making 3 batches of Rice Krispies to get each flavor.
Lightly coat an 8×11.5-inch baking pan with nonstick cooking spray or butter. Set aside.
Make the vanilla layer first. Melt 3 tablespoons salted butter in a large saucepan over low heat.
Add 4 cups miniature marshmallows and stir constantly until completely melted and smooth. Remove the saucepan from the heat.
Add 6 cups Rice Krispies cereal and gently stir until all the cereal is evenly coated with the marshmallow mixture.
Transfer the mixture to the prepared baking pan. Lightly coat a piece of wax paper with nonstick spray and use it to firmly press the mixture into an even layer.
Wash and dry the saucepan before making the chocolate layer.
For the chocolate layer, melt 3 tablespoons salted butter in the saucepan over low heat. Add 4 cups miniature marshmallows and stir constantly until completely melted. Remove from the heat.
Add 6 cups Cocoa Krispies cereal and gently stir until all the cereal is evenly coated.
Carefully spread the chocolate mixture over the vanilla layer. Use lightly greased wax paper to press it down firmly and evenly.
Wash and dry the saucepan before making the strawberry layer.
Melt the remaining 3 tablespoons salted butter over low heat. Add the remaining 4 cups miniature marshmallows and stir constantly until melted and smooth.
Remove from the heat and stir in 1 (3-ounce) box strawberry Jell-O until completely dissolved and evenly incorporated.
Add the remaining 6 cups Rice Krispies cereal and gently stir until all the cereal is coated with the strawberry marshmallow mixture.
Carefully spread the strawberry mixture over the chocolate layer. Use lightly greased wax paper to press it into an even layer.
Allow the treats to cool completely before cutting into 9 servings and serving.