Toss the gruyere, fontina and parmesan cheeses with the flour. Keep the mozzarella seperate.
Heat the butter and milk or cream in a large heavy bottomed saucepan until the butter is melted. Gradually add the three combined cheeses bit by bit and continue stirring gently with a wooden spoon until you have a smooth sauce. Keep hot over a low flame while you cook the pasta.
Cook pasta according to packet instructions until it is al dente. Drain and immediately transfer to a large serving bowl.Pour over the cheese sauce and stir quickly, then stir in the mozzarella and serve immediately on heated plates.