Question for expert about voice/ singing?

You seem to confuse singing flat (being off-pitch) with singing in a key signature that has a flat, such as E-flat major, which has three flats.

Singing off-pitch--whether the sung note is above or below the actual pitch--means that the singer is a quarter-tone off-pitch. Any singer who is off-pitch by more than just a quarter-tone isn't singing on pitch at all, but just guessing at the pitch.

(Actress Lucille Ball portrayed Lucy Ricardo on TV's "I Love Lucy," for which Lucy Ricardo made vague attempts to sing a note on pitch before singing the correct pitch.)

The following link is a table of key signatures, arranged by the number of sharps or flats in each key signature:

http://www.music.vt.edu/musicdictionary/appendix/k...

Here is a video clip about how to read music notes:

http://www.youtube.com/watch?v=wNqQqvT43yo