Pop the popcorn, salt liberally, and place in a large bowl.
Melt 3 tablespoons of butter in a medium saucepan.
Once the butter is completely melted, add in 3 cups of mini marshmallows, stirring until the marshmallows are melted. Turn off the heat as soon as the last marshmallow has melted.
Slowly pour about 3/4 of the marshmallow mixture over the popcorn in the bowl and gently fold it into the popcorn with a spatula until most of it is covered in the melted marshmallow mixture.
Pour the marshmallow popcorn onto the prepared baking sheet in a single layer.
Drizzle the remaining marshmallow mixture over the popcorn.
Quickly sprinkle the candy and sprinkles over the popcorn.
Allow the popcorn to cool before breaking into pieces and serving.