They would have had a third bomb ready to drop around by August 17th or so. After that they would have had enough fissile material for around 3 per month.

So they could have dropped more. Would they? And would they drop them on cities and not, say, use them more tactically? We don't know. On August 10th, upon learning about the availability of more weapons, Truman explicitly ordered that no further atomic bombs would be dropped without his express permission. He told his cabinet at a meeting that day that "he had given orders to stop atomic bombing." According to someone at the meeting, "He said the the thought of wiping out another 100,000 people was too horrible. He didn't like the idea of killing, as he said, 'all those kids.'"

Would he have done it anyway? Or used it differently? Or what? We don't know. Because he didn't have to make that decision, in the end. And it was essentially up to him at that point, and nobody else. (There were various military folk who speculated about future use and targets, but again, Truman had seized the authority on the issue.) So you could imagine various different possibilities, when we are talking about the behavior of individuals. He did, on August 14th, right before the final surrender announcement, tell the British ambassador that he thought he might have to drop another on Tokyo if the Japanese did not surrender — but it's not clear whether he would have followed through on that (either in using it, or using it on Tokyo, or what have you).