Prepare Meat Sauce:
In microwave-safe 1-cup liquid measuring cup, heat 1/2 cup water in microwave oven on High 1 minute. Stir in porcini mushrooms; let stand 15 minutes.
Meanwhile, in 12-inch skillet, heat oil over medium heat until hot. Add garlic, carrots, and onion, and cook 15 minutes or until vegetables are tender, stirring occasionally. Increase heat to high. Add beef, pork, and veal, and cook 10 minutes or until meat is browned, breaking up meat with side of spoon.
While meat is browning, with slotted spoon, remove porcini mushrooms from soaking liquid, reserving liquid. Rinse mushrooms to remove any grit, then coarsely chop. Strain soaking liquid through sieve lined with paper towel.
Add wine to meat in skillet and heat to boiling; boil 2 to 3 minutes or until most of liquid evaporates. Stir in tomatoes with their juice, salt, and mushrooms with reserved soaking liquid; heat to boiling. Reduce heat to medium-low; simmer, uncovered, 20 minutes or until sauce thickens slightly, stirring occasionally.
Preheat oven to 375° F.
Prepare Béchamel Sauce:
In 2-quart saucepan, melt margarine over medium heat. With wire whisk, stir in flour, pepper, salt, and nutmeg, and cook 1 minute. Gradually add milk and heat to boiling, stirring frequently with whisk to prevent lumping. Remove saucepan from heat; set aside. In small bowl, stir Fontina and Parmesan until well combined.
Assemble lasagna:
Spoon 1 cup Meat Sauce evenly in bottom of 13" by 9" glass baking dish. Arrange 4 noodles over sauce, overlapping slightly and making sure they do not touch sides of dish. (If your package of lasagna noodles has only 12 noodles, use 3 noodles per layer and do not overlap them.) Spoon about 1 1/2 cups Meat Sauce over noodles. Spread about 2/3 cup Béchamel Sauce over meat; sprinkle with about 1/2 cup Cheese Filling. Repeat layering 3 more times, beginning with remaining noodles, then meat sauce, béchamel, and cheese.
Cover lasagna with foil, preferably nonstick, and bake 30 minutes. Remove foil and bake 20 minutes longer or until heated through and cheese is lightly browned. Let stand 10 minutes for easier serving.
