The first car has 3 turnpike he can take, the second car as 2 way or choosing a turnpike and the third one must take the same as the second one, so he only has 1 choice.
You end up with 3*2*1=3!=6.
Now since we've probability, we'll guess each car as the same %chance of going in each turnpike, so we must count how many possible way there is.
Every car as 3 choices so you end up with 3*3*3=27
So the probabily is 6/27=2/9
That look good to me. But I might be wrong, so be sure to double check