In most cases the third word, no matter how small, is still a third word...though if there was a huge differential in google results I might use the "of" under the assumption its that much more popular.
Voted for the 2 word version over 3 from domain name only viewpoint. In practice, be driven by whats in common usage, if xxxx of yyyy is what is commonly used rather than yyyyxxxx, i'd tend to go that way, or possibly with both to cover the bases.