The three second rule is as follows: When you walk into the room where the girl you are interested in is or when she arrives in the vicinity you must initiate conversation within three seconds so to convey spontaneous thought. If you do not follow it and initiate conversation after a couple of minutes she may be less talkative and less attracted by your likely prepared speech. Even if you come out with some shit it will likely be better than preparing something to say. She'll probably be able to tell.
*Shit here she is, here goes*.."eh Hey you're looking fine today girl..where have you been i've not seen you since yesterday"
"yeah! I was at my boyfriend's house"
"oh, ok, eh, *wtf nooo!* - eh, what u been up to then?"