Do the math. The gear you're missing with a 20 cog and the big ring is duplicated with the middle ring and a 15 or 16. With 27 speeds, you're going to have some other duplicates, too.
Most of us, unless we're racing in fast packs, have no use for an 11 cog or even a 12. Just buy a 12-21, available in Dura Ace or Ultegra, or 13-23, available in 105. Either one will give you a straightblock up to 19 teeth. Particulary with a triple crank, you shouldn't spend a lot of time cross-chaining in the big ring & 2 largest cogs anyway. The extreme chain angle puts a lot of wear & tear on the driveline. When you get that far up the cassette, it's time for a double shift to a smaller chainring. Besides, if you really need a triple crank, you might occasionally find that having that extra couple of teeth on the back more than offsets the coolness factor of a tight cassette.