The theme of this Burger King commercial is any 2 sandwiches for $6. Across the top of the slot machine you have examples of which sandwiches you can mix or match together. In each row there are 3 examples of mixing/matching. So, 2 sandwiches represent the number 6, hence 6-6-6.
Remember, your subconscious mind picks up on these patterns a lot faster than your conscious mind does.