dcm2niix converts these images correctly. These are Mammography XRays. Each 2D image measures the attenuation across the 3rd dimension. In this way, the depth information is collapsed (this is like a 2D CT or MRI Maximum Intensity Projection where the bright pixel is shown at each location regardless of depth). If I were you, I would view these with MRIcron. Open three instances of MRIcron, and load one image in each. The images can not combined, as they collapse space differently. If you want to view 3D data, you will need to acquire a CT or MRI scan rather than an XRay. For these 3D volumes, one can choose to view the image from any persepctive (e.g. axial, coronal, sagittal). In sum, this is not a limitation of dcm2niix but rather a limitation of your images.