How to make sure a 3 character filename contains one of 2 characters?

How to make sure a 3 character filename contains one of 2 characters?

So I have to search a directory for files that are of 3 character length.
That is easy, the problem is that at least 1 of the 3 characters needs to
be either a "B" or a "y". I know that [By] would make sure the character
is either a B or a y, however simply doing [By][By][By] would not suffice
since it is not a requirement for all 3 characters to be either B or y;
only 1 of them has to be. Please help.