The best batsman should play in a position where he will get the most beneficial conditions to bat for the longest period of time. That doesn't necessarily mean #3. If your batsmen is the best in the world against spin, and only 'pretty good' against pace, then he should play at #5. If its the other way around, then he should open or come in at #3.
It's about putting your best players in the best possible position to succeed. And the team will succeed if/when that player can score the most runs for you.
As for the original question, it depends on the team. If you have a settled opening combination, than a guy like Ponting is perfect because the opening partnership usually builds a start, and he takes it to the next step. He can play the anchor role, but he generally doesn't need to. Dravid is perfect for India because our opening is frequently unreliable, and we need a rock for people to bat around.
If Ponting were in India, I'd want him to either open, or bat down at #4-#5 - he wouldn't be the ideal #3 for us. And I'm sure Dravid wouldn't be the ideal #3 for Australia.