Depends how they are chosen, of course. If they are chosen from, say, a bag of balls with numbers on them and each ball is left out of the bag when chosen, then picking three different ones is a certainty. If you replace the ball after it has been chosen and pull another at random, then the odds of choosing three different ones are as Jim has explained.
Depends whether you mean 3 particular numbers or if you mean you pick one replace it and pick again then replace and pick again, and want the odds that there are no duplicate's picked.
Also depends whether there are other numbers in the pot but your just wanting odds of getting numbers 1 to 24.
Maybe your better off telling us the actual senario exactly