It depends on your needs and how you want your bike to fit.
The Kestrel is definately more of a road/tri bike, and Kestrel admist it.
The Cervelo P2 is a pure tri bike, meant to be ridden low and in the aero bars. It's probably the fastest, gram of drag for gram of drag of the three choices.
The Giant is also a pure tri bike, however the geometry is a little more relaxed than the Cervelo. It may be a better option if the Cervelo would have a stack of spacers to fit you.
One other option in your price range is the Felt S22, I would say it's aerodynamically almost the equal to the P2sl and offers a lot for $2000.