You solve this by determining which fractions can have common denominators.
1/3 = 2/6 and 2/3 = 4/6, so at 1/3 and 2/3 there will be tire and a cone.
1/2 = 3/6 so there will be a tire and a hurdle.
6/6 = 3/3 = 2/2 so at the end of the course there will be all three: a tire, a cone and a hurdle.