The chest is only 3 mm for three reasons. First, thinner neoprene means your lungs can expand. Second, since your lungs already have air it lifts you, and the 3 mm is just the right amount so you are swimming balanced. Third, the thinner material permits even more flexibility in the arms (which are 1.5 mm).
The Vengeance Fullsuit is the swimmer's wetsuit. The Vengeance was built from the ground up with features carefully constructed to provide the most output in a swim. Fully coated with the slickest possible nano-super composite skin, it maintains an industry leading .02 drag co-efficient allowing for the smoothest possible swim. Unparalleled design similar to the Vendetta Fullsuit, the arms and shoulders have 6 anatomically correct panels, allowing for uncompromised arm and shoulder mobility. This means the wetsuit does not combat energy output from the stroke, arms don't fatigue from resistance with the material and results in a faster, more efficient swim. The Vengeance was designed with chest and torso thickness at 3 mm, and 5 mm in both the front and back of the legs. This aligns the swimmer to be more streamlined in the water, allows for natural range of motion in the stroke, and creates a feeling of "swimming downstream."