Make the glaze: Mix ketchup, brown sugar, and cider vinegar together in a bowl until well combined.

Make the meatloaf: Preheat the oven to 350 degrees F (175 degrees C). Spray two 9x5-inch loaf pans with cooking spray or line with aluminum foil for easier cleanup.

Combine onion and bell pepper in a microwave-safe container. Cover and microwave until softened, 1 to 2 minutes. Set aside to cool.

Mix oats, milk, eggs, garlic, mustard, Worcestershire sauce, thyme, seasoned salt, black pepper, and hot sauce together in a large bowl until well combined. Stir in cooked onion and bell pepper. Add ground beef, pork, and veal. With gloved hands, work all ingredients together until completely mixed and uniform.

Pat 1/2 of the meatloaf mixture into each prepared loaf pan. Brush each loaf with 1/4 of the glaze; set remaining glaze aside.

Bake in the preheated oven for 50 minutes. Remove the pans from the oven and carefully drain the fat. Brush remaining glaze over loaves, then return to the oven and bake for 10 more minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).

Remove from the oven and let stand for 15 minutes before slicing.