Warm a skillet over a medium-low heat. Slice the doughnut in half across the middle and generously spread the cut sides with butter. Put one doughnut half, butter side down, into the hot skillet then layer the 3 cheese slices on top. Add the remaining doughnut half, butter side up. Cook until the underneath is brown and the cheese is melting, then flip over and press down with a large spatula. Cook until the cheese has fully melted, then serve hot.