The standard X-rays required before starting orthodontic treatment are a panoramic X-ray and a lateral cephalometric X-ray. In some cases, there's information we need in order to draw up an accurate treatment plan that these two X-rays can't provide. The orthodontist may then ask for a 3D scan of a particular area or of the whole jaw. This isn't a routine X-ray, because it involves a higher radiation dose, and it's only done when necessary. Such a scan is needed, for example, to locate impacted teeth – teeth that have stayed trapped in the bone – to prepare for placing dental implants (not orthodontic ones), before removing deeply impacted wisdom teeth (only in some cases), when there's a cyst in the bone, and so on.
This answer is for general information. Your own case needs an examination by an orthodontist.