Put the grits in a large heatproof bowl and cover with water by 3 inches. Stir, then carefully pour off the water. Stir in 4 cups fresh water and cover the bowl tightly with aluminum foil.
Fill a large wide pot halfway with water and bring to a boil. Set the bowl on top of the pot (do not let the bowl touch the water) and reduce the heat to medium; cook the grits, undisturbed, 1 hour. Uncover, add a heaping teaspoon of salt and stir until smooth. Re-cover and continue cooking, adjusting the heat as needed to keep the water at a simmer, until the grits are tender and creamy, about 1 more hour; stir until smooth. Keep warm until ready to serve.
Just before serving, preheat the broiler. Combine the cheddar, gruyere and gouda in a bowl; add all but 1 cup of the cheese mixture to the grits along with the butter and stir until melted. Transfer to a 2-quart flameproof baking dish and sprinkle with the reserved cheese. Broil until the cheese is golden and bubbly, 5 to 8 minutes. Let rest 5 minutes before serving.