Most important about Cairngorm 3 is that it doesn’t represent anymore an MVC framework.
Cairngorm 3 is moving towards best practices and recommendations, gathered by the Adobe Technical Services Organization and partners over the last five years. It also contains a set of tools and libraries that can be applied across frameworks.
Basically, Adobe took some time to analyze how Flex is used, how it behaves in various applications, and Cairngorm 3 aims to come up with solutions for the most common architectural matters.
To read more go to Cairngorm 3 page.