Glad to see I've sparked some conversation about this. I genuinely just wanted to know why I had them. :mog:
Who would rather have none?
It has to do with nerves. The same idea applies to wearing clothing for the most part. During the day you do not feel that you are wearing your shirt, but as soon as you think about it you can feel it.
Also third nipples grow on your milk ridge, which goes from your arm pit to inner leg/groin, if I recall correctly.