Most doctors don't do them, you can always ask but usually you have to go somewhere else and pay for a session, and you shouldn't wait too much longer the baby will get too big for you to see anything good enough
For both of my pregnancy I got the 3D ultrasound. First one I asked for it but it was kind of becoming to late. She was lodge down in my pelvis. This pregnancy the lady gave me one. What it looks like you need to ask for one. When you go in next ask for it, if you don't go for another one ask your doctor if there is any way they can let you get one and have the insurance are it was needed. (most insurance won't pay for it if it's not medically needed)