In a large bowl, mix together the ground ham, pork and beef, cracker crumbs, eggs, milk, and onion. Form 1/2 cupfuls of the meat into balls, and roll in flour. Place in a baking dish, and refrigerate for 2 hours or overnight.

Preheat the oven to 350 degrees F (175 degrees C). In a saucepan, combine the brown sugar, water, vinegar and mustard. Mix well, and bring to a boil. Pour over the ham balls before baking.

Bake for 1 hour and 20 minutes in the preheated oven, basting occasionally with the drippings.