In a small bowl, stir bread crumbs with milk until evenly combined.
Let sit 15 minutes, or while you prep other ingredients.
In a large bowl, use your hands to combine beef, pork, sausage,
onion, and garlic. Season with salt and pepper, then gently stir in
bread crumb mixture, eggs, Parmesan, and parsley until just
combined. Form mixture into 1" balls.
In a large high-sided skillet over medium heat, heat oil. Working in
batches, sear meatballs on all sides to develop a crust. Set
meatballs aside, reduce heat to medium-low, and add sauce to
skillet. Bring sauce to a simmer then immediately add meatballs back
to skillet. Cover and simmer until cooked through, about 8 minutes
more.