You actually have a routine scan around 18 weeks anywhere where they can usually find out the sex then for you. The 3D/4D is more for you to see bubs as a 3D image. They usually prefer to determine the sex by the 2D format. So its prob not really worth your while if thats the only reason you want the 3D iykwm.
Um as what to say to your doctor.. you should get a referral from your midwives or OB anyways around that time to get a scan done, as i said you have one at 18 weeks anyways for them to check out bubs heart and other inside stuff. So maybe ask at your next appointment?