Little Suzy has a tricycle (you know, with three wheels). She wants to travel 1 mile with the tricycle, but she wants to spare her tyres (you know the price on tricycle tyres just keep going up). She is taking 2 spare tyres.
Assume she knows when to exchange wheels so that each of them travels an equal distance. How far does each wheel travel at the end of one mile? How did she exchange the wheels?
You have 5 tires to ride 3 miles, so 0.6 miles per tire.
The solution that minimizes the number of exchange tires requires you to exchange one tire each 0.2 miles. Ady's solution minimizes the number of pit stops, but considering that little Suzy should be a lazy lady, probably she would be bored stopping for two tires exchange, so it is more likely she would prefer frequent pit stops, taking a snack and exchanging her tire. So, my suggestion would be begin with tires 1,2,3 and after 0.2 miles use 1,2,4, then 1,5,4, then 3,5,4 and finally 3,5,2. And I agree with Penny, each tire 'travels' one mile and rides 0.6 miles... :)