In a large bowl, mix beef, pork, veal, salt, pepper, eggs, onion and heavy cream. With moistened hands, shape the mixture into 1 inch balls. Roll the balls in the bread crumbs until all balls are well coated. Arrange in a single layer on a large, shallow baking sheet.
Melt 1/2 cup butter in a large saucepan over low heat. Stir in the flour. Gradually stir in the chicken broth. Continue stirring until thickened and bubbly, then blend in the sour cream and fresh dill.
Melt remaining butter in a small saucepan, and drizzle over the meatballs. Bake meatballs in the preheated oven 35 minutes, turning occasionally, until evenly browned.
Place meatballs in a chafing dish and cover with the sauce to serve.