15 weeks is iffy for guessing gender. Usually, if they guess boy it's more likely to be an accurate guess. If they guess girl, there is a chance it may not be correct because the boy parts can still look like girl parts at that point. If the tech guessed based on seeing actual girl parts instead of guessing based on the absence of boy parts, it more likely to be correct. We got a boy guess at 15 weeks, but it still looks an awful lot like a female u/s. I'm waiting until my a/s at 21 weeks for confirmation before spending a lot of money and time preparing for a boy. With DD, it was very clear at 16 weeks that she was all girl.
We have an ultrasound picture with the "3 lines". But when she was doing the ultrasound, she said "There are the legs..." and my husband and I both thought it was a boy until she said "As you can see, there is nothing between them." But we both thought something was there, although we are first time parents and never saw an ultrasound before.