It all depends on your scan clinic. Some (if not most? I don't know) have both. My regular scan place where I go for my 12w etc do the regular 2D and can flip to 3D on the same machine whenever they want. That means that when I do my 20w scan I will get 3D. But if your scan place doesn't have that ability then you have to go to a specific 3D scan place. These are usually not medical, they're more to try to capture the sex more than anything (if you couldn't see at your 20w scan). Obv this costs quite a bit. Source around though as it's not moderated so you can find some that are much cheaper than others.
Hope that made sense.