They'll tell you what they see...
3 lines? Ahaha I am lost there and I have see so many u/s...
Okay with a boy, if they are able to 100% sure see if it's a boy, you will see a penis, and two testicles on the screen! Sometimes some doctors like to print that out for you, Like crazy as it is, I have 7 4d ones of my sons penis and testies! Ahaha, and with a girl basically, they will show you between the legs, obviously there wont be anything if it's a girl! When you go in, just tell them, you want them to explain what your seeing, on if it's a boy or girl!
Really easy to tell, for the boy! But good luck on your u/s! :)