I had a 3d done with my last baby at 14 weeks for my ultrasound and she was able to tell right away i was having a boy and that is what i had :) i havent had a 3d with this one yet but at my first doctors appointment i was 15 weeks and 5 days and we were able to see that i am having little girl this time. All are different tho so if the tech said baby was not very cooperative i woyld use my next to confirm.
Or a blood test can tell, but they're quite expensive.
I would wait til your anatomy scan before buying anything.