Turn broiler to low, set rack about 8 inches from broiler and line a baking sheet(s) with parchment paper.
Place bread in a single layer on baking sheet(s).
In a small microwave bowl, melt butter, add garlic salt and parsley flakes. Stir.
Brush butter mixture over bread slices and top with all three cheeses.
Place in oven, with broiler set to low, and broil for about 4-5 minutes or until cheese is bubbly and starts to brown. Make sure to keep an eye on this as ovens vary and this can burn quickly.
Carefully remove from oven and enjoy!