I don’t get anything in the 3D window. What do I need to do?
You can show slice views in 3D by clicking the eye icon in the slice view controller.
You can show CT images using volume rendering by drag-and-dropping the image from the data tree into the 3D view.
You can display specific structures in 3D by segmenting them. For example, you can use TotalSegmentator extension to segment 100+ structures (all major bones, organs, larger vessels, etc.) fully automatically.