"Yes, there was indeed a third bomb forthcoming if Japan didn’t surrender after the second bomb was dropped. This third bomb, sometimes referred to as the second Fat Man or the Third Shot, was another plutonium-239 implosion bomb. "
Two types of nuclear weapons were developed at the Los Alamos wartime lab in an effort to help end World War II. Both would be released above Japan just days apart in August 1945, and just weeks after the July 16 Trinity test in the New Mexico desert.
www.lanl.gov
"Groves’s directive, written on July 23, 1945, just weeks before Little Boy and Fat Man were released, stated that additional bombs would be delivered for use against the Japanese targets once they became available. The plutonium production facilities at Hanford in Washington state continued to work at capacity. Production of materials for assembly of a third bomb was well underway when Japan officially surrendered on September 2, 1945."