Gyume and Gyoto monks do overtone chanting, and can use three notes. I can do that sometimes, too...it is part of a meditation. Don Campbell wrote a book about it, and also, check out David Hykes and the Harmonic Choir.
Don Campbell website:
http://www.mozarteffect.com/
Metal singers usually use diads, as opposed to triads. There is a lady in LA, Melissa Cross...here is a link to her site:
http://www.zenofscreaming.com/
@TSS : That's not bad, but you are straining your vocal chords. You should check out the things I just mentioned. To get two notes, you can lower your trachea after some practice. To get the third note...is usually a nasal overtone (higher harmonic), and you can sometimes just change the shape of your mouth to achieve it. Also, it helps if you have some mucous in your throat. (Gross, but seriously, you can get a higher note that way.)
Another nice link:
http://khoomei.com/types.htm
