In the bowl of a stand mixer, combine bread with milk, tossing to coat.
Let stand, tossing occasionally, until bread is completely moist, about 10 minutes.
Squeeze bread between your fingers or mash with a spoon to make sure there are no dry spots ( add more milk 1 tablespoon at a time until bread is moist throughout if needed)
Add onion, garlic, Parmigiano-Reggiano, parsley, salt, pepper, pancetta, egg yolks, and oregano to bread/milk mixture.
Set mixer bowl in stand mixer and attach paddle.
Starting at low speed and gradually increasing to medium-high speed, beat bread mixture until thoroughly blended, stopping to scrape down sides as necessary.
Add one third each of the beef , veal and pork and beat at medium-high speed until thoroughly blended with bread mixture.
Remove bowl from stand mixer and add remaining beef. veal and pork.
Using a clean hand, gently mix meatball mixture, teasing apart ground meat with your fingers, just until ground beef and pork and thoroughly distributed. (don't over mix)
Preheat broiler and set oven rack in upper position.
Form meatball mixture into handball-sized balls and arrange on prepared baking sheet covered with foil.
Broil meatballs until browned on top, about 5-7 minutes
Meanwhile, Heat tomato sauce in a medium pot until simmering and add meatballs.
Simmer until meatballs are just cooked through about 10-12 minutes