Rauenpc,
There is a lengthy discussion on this in the Aruba Outdoor MIMO Wireless Networks VRD on our web site, pages 63-66.
In general, it is difficult if not impossible to achieve 3SS rates outdoors. This is because there are only two usable antenna polarizations (H and V). All other polarizations, including +/-45 and circular are ultimately variations on H and V.
The reason polarization matters is that we do not generally have very much multipath outdoors, especially as compared with reflection-rich indoor environments. So to achieve the 2nd spatial stream we have to "create" multipath which we do with dual-pol antennas.
However, since there is no 3rd or higher-order disrete polarization, this same technique cannot be extended past 2SS.
Now, there is evidence to suggest that using a 3SS AP with a triple-pol antenna (H, V, +/-45) can produce a slight range benefit for a given rate. This is due to processing gain in the baseband on both sides of the link. But in practice you will only see 3SS rates for 20-50 meters past the AP in an outdoor setting before they fall back to 2SS.
-cl