Since there are 10 runners in all:
For the first position there 10 possibilities.
For the second position there are 9 possibilities, after the 1st has been taken.
For the third position there are 8 possibilities, after 1st and 2nd have already been taken.
Total = 10 * 9 * 8 = 720
So there are 720 ways.
I hope this helps.