In a large pot, bring water, milk and half and half to a simmer. Watch carefully so it doesn’t boil over. Add salt and slowly whisk in grits. Bring grits to a simmer, stirring constantly. Lower heat and cover grits. Cook 45-50 minutes, stirring occasionally, until grits are rich and creamy. Stir in butter. Pour grits into a 9 x 13 baking dish and let cool to room temperature.