Preheat the oven to 350°F. Spray a 9x13 inch baking dish with nonstick cooking spray; set aside.
Mix together ham (1 ¼ pounds), ground pork (1 pound), and ground beef (1 pound) in a large bowl. Stir in milk (1 cup), eggs (2), and graham cracker crumbs (1 ½ cups) until well combined.
Using your hands, form the meat mixture into large balls with 1/3 cup filling, about 2 inches in diameter, and place them in the prepared baking dish.
In a medium mixing bowl, stir together tomato soup (1 (10.75-ounce) can), vinegar (¼ cup), minced onion (1/4 cup), brown sugar (1 cup), mustard powder (1 teaspoon), and Dijon mustard (1 teaspoon). Mix well and pour over the meatballs.
Bake in the preheated oven, for about 1 hour, or until an instant-read thermometer reaches 160°F.
Notes
Mixing: Obviously the meatball mixture, as with any meatball mixture, must be completely and thoroughly combined. That said, if you overmix the mixture, you run the risk of tough meatballs.
Using a Large Cookie Scoop: generally measures approximately 1/4 cup. This recipe calls for the meatballs to be measured at about 1/3 cup before baking. You can just use a heaping scoop to measure.