Explanation:
a row n adds the number n to the sum (= the first number from the right in row n-1), and therefore also n to the 3rd number from the right of row n-1.
in any case, the first number from the right is the result of
1 + 2 + 3 + 4 + 5 + ... + n-1 + n
that sum is
n(n+1)/2
for the row 20 that is
20×21/2 = 10×21 = 210
the 3rd number from the right in row 20 is then
210 - 2 = 208