In a blender or food processor, pulse the Fruity Pebbles cereal into fine crumbs.
Combine the Fruity Pebbles cereal and 2 cups of frosting and stir until combined. The filling will now be a little thicker than the frosting. It will spread more like a cookie dough filling. I suggest making the filling when you're ready to use it.
Place your cake board on a turntable. Spread a small dollop of frosting on the cake board and place the first cake layer, top side up, on the center of the cake board.
Carefully spread half of the Fruity Pebbles filling on the cake layer, followed by 1/2 cup to 1 cup of the Fruity Pebbles Crunch.
Place the second cake layer, top side up, on the fillings and repeat step 2.
Place the final cake layer, top side down, on the second layer of fillings. With the vanilla frosting, lightly frost the cake to create a crumb coat. (This layer of frosting is meant to be thin and does not need to be perfect. The point of the crumb coat is simply to lock in the crumbs.) Freeze the cake for 10 minutes to set the crumb coat.
Continue frosting the cake with the remaining frosting and extra Fruity Pebbles Crunch. Best served day of assembly. Can also be frozen in an airtight container for up to a week.