The Scott & Giant have the best components, with the Scott upgraded to Ultegra shifters and the Giant having the shorter-reach shifters. The Specialized has carbon seat stays, which are supposed to help with ride quality, but has lesser-quality 9-speed components. I've personally never ridden an alu bike with carbon seat stays, so I have no idea if they work.

If you can't feel a difference, I'd probably go for the Scott because of the substantially better components and it's prettier than the Giant.

This is assuming, of course, they're all equally nice to ride.