Any diversity or 802.11n multi-packaged antenna is simply 2 or 3 antennas in a single package. So there's nothing special about them, and simply purchasing three identical antennas is the same as using one of them.
I guess that depends by what you mean when you say diversity.
Directional antennas provide just that, direction. Where your signal coverage is 'directed' to just a certain area, such as long range, or in a 90 degree pattern, etc.
By diversity, that sounds more like the fact that each radio has 3 antenna outputs. So each of the radios in the 1252 Access Point is sending out 3 signals at one time. Whether each of the these 3 antennas is separate, or all built into one piece..its still 3 diverse signals.
Hope this doesn't confuse you any more than you already were :)