According to Wikipedia: "The SM-65E Atlas, or Atlas-E, was the first 3-engine operational variant of the Atlas missile, the third engine resulting from splitting the two booster thrust chambers into separate engines with independent sets of turbopumps. It first flew on 11 October 1960, and was deployed as an operational ICBM from September 1961 until March 1965. Following retirement as an ICBM, the Atlas-E, along with the Atlas-F, was refurbished for orbital launches as the Atlas E/F. The last Atlas E/F launch was conducted on 24 March 1995, using a rocket which had originally been built as an Atlas-E."