In a blender or food processor put the tuna, anchovies, and garlic on low speed. Add olive oil until you get a smooth paste.
When the stuffing mixture is done start filling the olives one by one. You can use a piping bag, a teaspoon or a sandwich baggie with a corner clipped. Any which method will do the trick.
Watch them disappear. You can use any extra filling on crackers, or bread. YUM!