The differences between Brooks Ravenna 3 and Ravenna 2 primarily involve volume and cushion. Version 3 is deeper, has more volume and fits less snugly than version 2. Ravenna 3 also has a softer midsole, which is great for cushioning but not as durable as version 2.
Overall, Ravenna 3 is a great shoe, however because of the changes with version 3, you will want to try on and confirm fit prior to purchasing. Video comparison between the two versions can be found here:
Here’s my previous post on Brooks
Ravenna 2: http://drshoe.wordpress.com/2011/10/01/shoe-review-brooks-ravenna-2/