Why didn't Japan surrender after the first atomic bomb?
Three days wasn't enough time because shortly after the first atomic bombing, Manchuria was invaded by the Russians. Manchuria was Japan's line of communications to the Allies. This prevented the Japanese from communicating with the US until after the bomb was dropped. Little known fact, Stalin was making contact with the Japanese well before this for the Japanese to conditionally surrender, although Stalin kept this hidden from the other Allies. And when the bomb had gone off in Hiroshima, there was no one there(obviously) to let the Japanese government know. Japan's government officials were under the opinion that it was just some heavy duty bombing at first, and when they later heard it was an atomic bomb, asked their team of scientists who were working on their own bomb if it was possible if the Americans had more than one. The team claimed they didn't, but the Japanese still debated amongst themselves if they should surrender under the American's terms of surrender(which were unconditional). If you are referring to the atomic bomb dropped on Hiroshima, there are several reasons for Japan's continued resistance after its devastation of that city.
First: Some Japanese leaders suspected the bomb might have been a one-time trick. By continuing resistance, they were "calling our bluff," or so they thought.
Second: The Japanese believed that they could negotiate a favorable armistice with the Allies using the Soviet Union as a third party. When the USSR declared war on Japan on August 8, this went out the window and Japan had nowhere left to turn.
Third (tying back in with #1): The Japanese needed proof that the United States had or was building up a nuclear arsenal for use on them. When the second bomb obliterated Nagasaki, it became apparent that the US would not invade and incur massive casualties but "just keep dropping atomic bombs." To the Japanese General Staff, the bombs represented the ultimate American trump card against their plans for a "decisive battle:" rather than negotiating with the Americans after the latter suffered huge losses in the Home Islands, the US would bomb Japan into submission at no cost to themselves.