The Castelli Core Mesh 3 Sleeveless is designed for use in the heat of the summer months.
By wicking away sweat and encouraging the circulation of air between your skin and jersey, it allows your body to maintain a more constant temperature, so that you'll stay comfortable and perform better.
The big upgrade on the 3 model is a more open neck for improved cooling (it also sits under an aero jersey better) and the use of a new stretch Magic Mesh that fits snugly but does not contain Lycra.