I don't know if there was enough Plutonium for a thrid bomb. From what I've understood they US only had enough for 3 bombs and it took them nearly a year to make more.

Now as too the bombs use. Well the US governemnt was pridicting 1 million US casualties to invade the home islands of Japan. They were convinced it would be Okinawa on larger scale. They wanted to shock and awe the Japanese into swallowing there pride and surrendering.