I know someone who got it wrong (at the 12 week) and when they found out at the 20 week my friend was devestated. It's not she wanted one sex more, but she'd bonded and basically planned her future (as you do). She had 20 weeks to adjust, but I can understand her shock.

I would get another u/s if I was you. To me that is the most unclear thing I've ever heard. Im sorry for you that he didn't spend another few minutes having a good look for the three lines. xo