gender prediction: the 3-line test in a sonogram...is it accurate to predict a girl?

i went in for my 20 week scan and the baby wasn't cooperating at all with anything they were trying to do...including trying to figure out tha sex. they scheduled another appt at about 22 weeks and the tech said then 'she thinks its a girl'. now when she said it, she showed me on the screen what she believed to be 3 lines between the legs, looking from underneath the bum-bum. i know that this is the sign the techs look for in determining a girl, but i want to know how accurate it is. and by accurate i mean if they see these infamous 3 lines, is there any chance they can be mistaken for the bottom view of a boy's parts. Has anyone ever been told this and ended up having a boy? or are they mostly accurate if they do see these lines?? i'm starting to buy pink, and alot of it so i want to be sure...