Hooking up a single antenna will not give the desired performance, and may in fact be detrimental to performance. Draft N uses constructive interference to achieve it's high data rates, this requires a matched antennae array that should be aligned in a particular fashion. A single antenna, regardless of which lead it is hooked to can not provide this effect. At best you will be getting 802.11g at any distance that would be greater than the reach of the other 2 antennae.
To those trying to hook up the D-Link Draft N Antennae Array,
Yes, just connect the three ends of the antennae to the three leads on the card or router.