Perfect example of the problem I suffer: Shimano 105 St 5600 Double Triple 10 Speed Shifters Set Flight Deck VG Light Use | eBay

This guy says (I PM'd him for specifics) these 5600s came off his '07 Trek Portland, and are most definitely 5600s and most definitely accommodate 3 rings.

So I'm going with the theory that Shimano changed their design in the middle of a generation.