Puzzling

Here is a good riddle:

There are 25 runners. Only five can run at any given time. You don’t have a stopwatch, or any other means of timing the runners. Assume they all look relatively similar; you can’t tell if one person is faster just by looking at them.

How many races do you need to run to determine the three fastest runners?

I’ll see if I can figure it out today, but let me know if you crack it first!