Since your a Ham I can make this simple, the encoding done on data on a draft N connection is based on multiple send and receive streams, and is adjusted for constructive interference between the multiple transmit antennas. a single antenna will not allow you to connect at N speeds.
An amplifier can help, however they are of questionable legality, I am no master of the FCC regulation on transmission power, but I believe some of these products may violate their regulations. And as I recall the regulations are limited by gain and power so you might be able to push a certain wattage, but not have the kind of antenna array to receive.
Ideally what you would need is an array designed with the draft N multiplexing in mind, and if you need the power you will need it on all 3 antennas.
However much like the equipment you work with more power isn't always the best option, with WiFi it is usually external interference and your own reflections that make the most difficulty for you.