We can choose the first letter 3 ways,
For each of those 3 ways to choose the first letter, there
are 2 ways left for us to choose the second letter.
And there is only 1 letter left to choose the third letter.
That's 3 = 6 ways.
Checking:
boy, byo, oby, oyb, ybo, yob
Count them. That's 6 ways.
Edwin