Preheat the oven to 200 degrees F and coat a rimmed baking sheet with cooking spray. Place the popcorn, pretzels and cashews in a large brown paper bag and shake to combine; set aside.
Microwave the butter in a large microwave-safe bowl until melted, about 1 minute. Stir in the brown sugar and corn syrup and microwave until the mixture begins to boil, about 2 minutes. Carefully stir, then microwave 2 more minutes.
Add the baking soda to the sugar mixture and stir until it begins to foam. Carefully pour over the popcorn mixture in the bag. Fold over the top of the bag and shake to coat the popcorn. Microwave in the bag, 1 1/2 more minutes.
Pour the popcorn mixture onto the prepared baking sheet and bake 1 hour, stirring every 15 minutes. Let cool completely before serving.