To talk Economic bs, demand is not directly changed.
As 3 char's all get registered, the supply of cheap 3 char .com's drops, so people looking to having 3 char's will have to settle for higher prices.
However, with higher prices, demand is in turn increased, because as domainers see that 3 char .com's rise in value, more and more people are demanding them to resell and gain some profit.
So, in turn, the price will rise even more, and even more domainers will demand them.
This cycle alone cannot be sustained. However, this cycle will lead to more 3 char .com's being developed, and many of them may get successful and their owners become rich.(note I'm saying some will become rich, many will fail, but that doesn't matter, people don't notice them much) Then, 3 char .com's become a known money-maker, and the demand for them will become even higher.
IMO, the value of 3 char com's will keep rising, until someday should there be another .com bust.