Line a 9" x 13" baking pan with parchment paper and then spray the parchment paper with cooking spray. Set aside.
In a large Dutch oven or pot, add butter and melt over low heat. Next, add two bags of marshmallows, stirring continuously until smooth and melted. As soon as the marshmallows are melted, remove the pot from the heat and then stir in the salt and vanilla.
To the melted marshmallow mixture, add the Rice Krispies, Fruity Pebbles, and the last bag of marshmallows. With a sturdy spoon, stir until the cereal is evenly dispersed throughout the marshmallow.
Transfer the mixture to the prepared pan. Coat an offset spatula with cooking spray and use it to gently spread the mixture in the pan in an even layer. Allow the mixture to cool completely and set in the pan (about one hour) before slicing into squares.