In a medium bowl, combine all cheeses. Divide the cheese mixture evenly between three slices of the bread, top with the remaining three bread slices to form sandwiches, and butter the outside of the bread.
Heat a well-seasoned cast-iron skillet or any large pan with a heavy bottom over medium heat.
Place a sandwich in the heated pan and cook until the bread touching the pan is golden brown, 5 to 7 minutes. Flip the sandwich and continue to cook until the cheese is completely melted and the bottom of the sandwich is golden brown. Repeat for each sandwich and serve right away.
Makes three sandwiches