Out of the bicycles mentioned, I would recommend the Specialized Hardrock comp disc, personally. Reasoning?
Crankset has replaceable chainrings. Both the Trek and Cannondale are using a Shimano Altus crankset which does not have replaceable chainrings.
The Hardrock has a lockout on the fork, which you can use while road riding. The Cannondale does not. I'm not sure if the Trek does. Interestingly, the Trek 4300 has a different fork in its picture on the website than what the text below that picture says should be there. What's going on, Cannondale?
The Hardrock is equipped with SRAM X5 for a rear derailleur, SRAM's equivalent to Deore (higher in quality than both the Acera on the Cannondale and the Alivio on the Trek)
Hardrock has Avid BB5, which isn't a top notch brake, but it's pretty decent for the price tag. This is more of an opinion thing, but I prefer these over the Tektro IO and Shimano 416 (Alivio grade) on the Trek and Cannondale
Lastly, opinion again, I think the Specialized Hardrock just looks cooler than the other two. It's got a nice smooth sweep on the top that's just a bit nifty, and the colour scheme on the bike is nice.
I'd personally recommend giving them all a test ride and seeing if one really fits well and stands out, of course.
But if it were a matter of picking one for the features, so long as the price was relatively level, I think that the Hardrock is the pick for me.