To see the 3rd nuke coming, you must have Kyujo Incident succeeded, so that Japan decided not to surrender, while the Emperor would be put under house arrest. The third target would obviously not be Tokyo as suggested by the All Evil Wikipedia, after all it would not be wise to nuke the capital of Japan. The United States intended to nuke Kokura instead of Nagasaki on August 9, so it may well be a possibility for the third target. If there are other choices, they would be Sapporo, Hakodate, Kyoto or even Osaka. If Osaka got nuked, then the reconstruction would be far more difficult, and Japan would not possibly recover until the early 1960s; while nuking Kyoto would mean a kind of cultural genocide, which would have extremely negative effects as well. Depends on the Soviet progress or plan to invade Hokkaido, the United States may very well try on Sapporo, Hokkaido.