I’ve searched the web about .dts files and it seems that old versions of Philps iSite software were only able to save files in proprietary format that only the iSite viewer can use. Newer versions can also save DICOM files. If maximum image quality and reliability of physical size is important then I would recommend to request another export of the data set, in DICOM format.
Acquisition of 3 highly anisotropic MRI volumes is quite common, as they are well suited for humans reviewing them on 2D screens. However, they are barely usable for 3D reconstruction and analysis. See more details in this post: Combining volumes - what am I missing?