Line your pan with 3 cups small pretzel twists plus ¼ cup for garnish. On a 9x13 sheet pan lined with parchment paper, lay out your pretzels in a single layer as tightly together as you can. They don’t have to reach the sides of the pan.
In 3 separate medium heat-safe bowls, heat the melting chocolates according to package instructions. Do not overheat the melting chocolates. They’ll seize, and you’ll be left with a lumpy, crumbly texture. You also want to make sure your bowls are free of any water droplets because it will affect how the chocolate melts.
Pour 2 cups white melting chocolate over the pretzels and spread it out to the edges of the pretzel, making sure to fill every gap between them.
Drizzle 1 cup blue melting chocolate and 1 cup red melting chocolate all over the white chocolate. Move quickly before the chocolate begins to set. Use a toothpick or the point of a knife to swirl through the colors, creating swirls and lines into the white chocolate.
Once you are happy with the design, sprinkle the chocolates with ¼ cup red M&Ms, ¼ cup blue M&Ms, ¼ cup mini marshmallows, extra pretzels, and 3 tablespoons red, white, and blue sprinkles
Place the pan aside at room temperature or in the refrigerator to set for about an hour.
Once the chocolate has hardened, use a large sharp knife to cut pieces, or you can also break it apart with your hands to create uneven “broken” pieces of pretzel bark. Serve room temperature or cold.