Preheat oven to 400°. In a large bowl, combine egg, bread crumbs, salsa and onion. Add turkey, pork and beef; mix lightly but thoroughly. Shape into 1-1/2-in. balls. Place meatballs on a greased rack in two 15x10x1-in. baking pans. Bake until browned, 18-22 minutes.
In a 6-qt. slow cooker, combine sauce ingredients. Add meatballs; gently stir to coat. Cook, covered, on low until meatballs are cooked through, 3-4 hours. Freeze Option: Freeze cooled meatball mixture in freezer containers. To use, partially thaw in refrigerator overnight. Microwave, covered, on high in a microwave-safe dish until heated through, gently stirring and adding a little water if necessary.