In a large mixing bowl, add ground turkey, pork and beef, Italian sausage (no casing, just the meat), bell pepper, onion, garlic, eggs (beaten), crushed Ritz crackers (or plain bread crumbs), Worcestershire sauce, dried oregano, dried red pepper flakes, salt and pepper.
With clean hands, mix together until well combined. On a large plate, form the mixture into the shape of a loaf that will fit inside of the slow cooker without touching the sides.
To the bottom of the slow cooker pot, add olive oil (this is added so that the meatloaf doesn't stick to the bottom of the pot). Tilt the pot from side to side, or use a spatula to spread the oil so that the bottom of the pot is evenly coated with the oil.
Transfer the meatloaf from the plate into the pot. If needed, slightly reshape the sides of the meatloaf so that it's not touching the sides of the pot.
Along the sides of the meatloaf, add water. Cover with a lid and cook on the low setting for 7 hours or until the meatloaf is thoroughly cooked (depending on the thickness of the loaf it may take longer than 7 hours when cooked on the low setting). About 30 minutes before the meatloaf is done cooking, make the tomato sauce topping.