Preheat the oven to 350 degrees F. Spray an 8 1/2-by-4 1/2-inch loaf pan with cooking spray. Line the pan lengthwise with a long strip of parchment, rubbing it to adhere it to the pan and making sure it is long enough to hang over the ends of the pan.
Put the beef, lamb and pork into a large mixing bowl and add the olives, onions, crushed saltines, yogurt, mint, oregano, cumin, garlic and egg. Add the hot sauce, salt and some ground pepper and very gently mix with your hands until the ingredients are evenly incorporated. Add the feta and mix lightly, incorporating the feta in whole crumbles if possible. Try not to overmix; it can make the meatloaf tough.
Transfer the meat mixture to the pan and press it gently to eliminate any air pockets. Set the loaf pan on a baking sheet and bake until an instant-read thermometer inserted into the center registers 155 degrees F, 40 to 45 minutes.
Let the meatloaf cool in the pan for about 20 minutes before using the parchment paper to lift it out of the pan and transfer it to a serving platter. Slice while warm and serve with the Mint Tzatziki on the side.
Grate the cucumber on the large holes of a box grater into a large fine-mesh strainer set over a bowl. Sprinkle the cucumber generously with salt, tossing a few times with your hands to evenly distribute it; let stand for 10 minutes to drain.
Using your hands, squeeze the cucumber to drain any excess moisture. Transfer to a mixing bowl and add the yogurt, mint and garlic. Stir well to combine, taste and adjust the seasoning with salt if necessary.
Store the tzatziki in an airtight container for up to 3 days.