Is the DEXA scan 3D?

No, there is no such thing as a 3D DEXA scan.

Like most X-ray modalities, a DEXA body (and bone) scan produces a two-dimensional image and the X-rays scan you in one plane.

A CT scan (and an MRI, which uses radio waves, not X-rays) uses the familiar, big round ‘doughnut’ which circles around your body to fire X-rays from every angle. They might be said to be ‘3D scans’ and can produce images of cross-sections of parts of your body.