Why did Dean use the exponent 3 in Line 4?

_ _ _ here are the three numbers.

Since we know that the first one has to be 3, there is only one choice for the first number

1 _ _
Let's find the total number of ways first.
The next two digits have no restrictions except for being distinct, so it is 
1*9*8=72 total ways

Now ways with odd
still 1 choice for the first digit (1, 5, 7, 9) 4 choices for the second and 8 for the third.

32/72=16/36=8/18=4/9 as the probability=A