Hey there!
Since there are 3 friends running an equal amount of this relay race, each will run 1/3 of the relay.
This is because 3 thirds make a whole, and each third is equal to the others, so if you add each distance ran by each of the friends, or simply multiply each portion by 3, you'd get 10 miles.
If you're also looking for how long each friend runs, which I know isn't in the question but I'll add it anyways, it is 3 1/3 miles. This can be found by dividing 10 into 3 equal parts.
Hope this helps!