17 weeks could they be wrong about the sex?

It's too early to tell.  They could tell at approximately 30 weeks gestation.

by 20 weeks they should be able to tell....at 17 weeks they can be pretty accurate though...development of genitalia begins somewhere around 15 weeks you will most likely have another ultrasound throughout your pregnancy...but it isnt often they will give you a sex if they are unsure

The only way to find out definatively is via amniocentisis.   sonograms are definately not definative particularly with girls.....boys stand a better chance at being able to tell  with a bit more chance.

i would wait and get another ultrasound to see if theyre right. they told me mine was a girl and i went back before my due date and then they told me it was a boy. 

I have had a tremendous amount of ultrasounds during my twin pregnancy with two girls.  Three lines tells if it is a girl determining the presence of a vagina and vulva and this can be seen at 18-20 weeks.  I ask the technicians a lot of questions and they always responded with the above answer.  My son, on the other hand, defitnitely had a penis between his legs which was quite obvious.