The probability of having letter A is (1/26), the probability of having B (1/26) and the probability of having letter C is (1/26)
We can only have one ABC in that order.
To find the total probability we have to multiply each of them.
So:
[tex]\begin{gathered} \frac{1}{26}\lparen\frac{1}{26})\lparen\frac{1}{26}) \\ =\text{ }\frac{1}{17576} \end{gathered}[/tex]The probability is 1/17576