When would you use the Nikon 20mm 2.8, the 24. 2.8 and the 28mm 2.8?

What's the purpose and which is most versatile. Focusing on travel and street using the D7000

Landscapes, interiors, architecture, interiors.
Lens choice depending on width requirements of the shot.

I'd get the 14-24mm f2.8 or 16-35mm f4.0, and you'll have those focal lengths covered.

For the street and a little landscape, you may survive with just the 24mm. If you expect to travel to places with really expansive landscapes, the 20mm will be better. For general purpose shooting, the 28mm would do just fine.

http://keerok-photography.blogspot.com/2011/05/lenses-so-many-of-them-there-is-no-best.html