In a Dutch oven or large, heavy flameproof casserole dish over medium-low heat, melt 2 Tbs. of the butter with the olive oil. Add the pancetta and sauté until just starting to crisp, about 3 minutes. Add the onion, carrot and celery and sauté until aromatic and very lightly golden, 8 to 10 minutes. Add the ground beef, pork and veal, stir to break up, season with salt and pepper, and add a generous pinch of nutmeg. Increase the heat to medium and sauté until the meat is lightly browned. Add the wine and simmer until it is almost evaporated.