In 5D, the defense gets three trumps. It's hard to see how without a little help from the defense. Declarer has 6 top tricks, with a two more which can set be up in spades and one more in clubs, or, even more slowly, in his long diamond suit.
Timing is difficult, however.
For example, on a club lead, one entry to the south hand is lost, and when declarer leads a spade from north, east covers with the ten, which blocks the spade suit. The heart ace is still available for one more diamond lead off dummy, and you finally get to ruff with your three. You score all four trumps in the south hand, a club ruff in north, and five top tricks in the other suits. Amusingly, the three defensive tricks are west's remaining trumps.