Probability of getting 1 distinct unit in 3 selections means in all the 3 selections, only 1 particular element is drawn.
Now, for the first selection, whatever element we choose it will be distinct. So the probability is N/N = 1.
For 2nd selection, that particular element (which was drawn in the 1st selection) has to be drawn out of N elements. So the probability is 1/N.
Similarly for 3rd selection, the probability will also be 1/N.
So the final probability becomes the product of the above three --- which is 1/N^2.
try to find your answer now.