The Atlas III vehicle is an improved version of the established Atlas II family of launch vehicles.  The prime modification was replacing the two Rocketdyne MA-5 first stage and single Rocketdyne MA-5 sustainer engines with a single NPO Energomash RD-180.  Furthermore, Lockheed Martin simplified vehicle construction by drastically reducing the total part count. 

Two versions of the Atlas III are currently available.  The Atlas IIIA has a single RL-10A-4-2 engine powering the Centaur upper stage while the Atlas IIIB has two RL-10A-4-2 engines and a stretched Centaur upper stage to increase GTO performance to just under 10,000 lb.  Both vehicles are being marketed by International Launch Services (ILS).