There are 25 horses and you need to figure out the three fastest horses by placing them into races. Assume there is no tie in the speed. There are five tracks so for each race, you can place five horses and figure out the relative rank among those five horses but you don't have the exact finishing time, i.e. there is no direct comparison between results from two different races. What's the minimum number of races you need to arrange in order to figure out the three fastest horses?