I can tell you that the 3 is in another class. the carbon fork, disc brakes, shifters, crankset,tires and wheels are a big plus over the 6 and my 7. I rode a Giant Escape and while it was a very nice bike it didn't fit my build like the Cannondale. Not a lot of Cannondale Quick people here as the Trek bikes have a big following on this forum. That said that is another good bike you might want to try but you said you already have the 6 coming in for you so might be to late. Trek makes a nice bike but again just not a comfortable fit for me.
The 3 is my version of a road bike as I can not deal with the drop bars due to my back. Not trying to feed you the Cannondale kool aid or drive you away Just get a bike that is comfortable for you so you can spend time enjoying the ride.
Good luck, Frank.