Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper. Make the meatloaf: Heat a medium skillet over medium heat. Add the olive oil, then the onion and sauté until translucent, 3 to 5 minutes. Season with salt, add the garlic and sauté for 1 to 2 minutes without letting the garlic brown. Set aside to cool.
In a large bowl, whisk the milk, mustard, Worcestershire sauce, hot sauce and eggs, then season with salt and pepper. Add the beef, pork and veal along with the crackers, parsley and cooked onion-garlic mixture. Use your hands to mix until everything is evenly combined.
Place the meat mixture on the prepared baking sheet and use your hands to gently form it into a rustic loaf shape, about 9 by 5 inches.
Make the glaze: In a small bowl, combine the barbecue sauce, brown sugar, vinegar and Sriracha. Slather the glaze all over the meatloaf and bake, basting once halfway through, until the internal temperature registers 165 degrees F, about 1 hour 10 minutes. Let rest 20 minutes before serving.
Before baking the whole meatloaf, cook a small patty and taste for seasoning.