Number the cards from 1 to 26, A thru Z.

Then if they are in numerical order they
are also in alphabetical order.

Of the 3 numbers you pick, there is always 
a smallest number, a medium-sized number, 
and a largest number.

There are 3!=6 ways to order "small, medium and large", 

Each of these are equally likely.

1. Small, Medium, Large
2. Small, Large, Medium
3. Medium, Small, Large
4. Medium, Large, Small
5. Large, Small, Medium
6. Large, Medium, Small

So the answer is 1/6.

[The probability would still be 1/6 regardless 
of the number of cards in the deck, as long as 
there's 6 or more.] 

Edwin