1) Because 3 is not enough. You give your opponents too good implies odds to hit better hands and will have problems to get max value postflop.
A general discussion
3) There are ways to get away from the hand, like when he just calls your cbet, the turn comes 8[img]/images/graemlins/spade.gif[/img] and villain gives you much action. But you are not going to get away from a big overpair on this drawy flop w/o specific reads because he might play back at you with many worse hands than sets.
We want you to bet more because we want you to get the most value in case villain holds a T or 9 and to protect your hand in case he has a draw.