Heat oil in a large heavy pot over medium-high heat. Add onions, celery and carrots and sauté until soft, about 8 minutes. Add beef, pork and pancetta and continue sautéing, breaking up the meat as it cooks to a small crumble, until browned, another 15 minutes.
Add wine and boil for 1 minute, stirring and scraping up any browned bits. Add 2½ cups chicken stock and tomato paste, stir to combine. Reduce heat to very low and cook at a gentle simmer, stirring occasionally, for 1½ hours. Season with salt and pepper.
In a small saucepan, bring milk to a simmer. Gradually add hot milk to the sauce, a little at a time and stirring to blend. Cover with lid slightly ajar and simmer another 45 minutes until the milk is completely absorbed. If needed, add more stock by ¼-cupfuls to thin (I did not find this necessary).
Cook pasta in large pot of salted water until 1 minute shy of al dente. Drain, reserving ½ cup of pasta cooking water. Transfer ragù to large skillet over medium-high heat. Add pasta and toss to coat. Add reserved pasta water, a little at a time, as necessary, if it seems too dry. Plate and serve with freshly grated Parmesan cheese.