hey that's cool... curiosity, i get that
i didn't want to end up trying to get u some info you don't need to know. i mean basically i don't have full answers or the 3d tools etc but i think i can help some
i think you can use a couple of apps to view these files. when i discussed recently, this was for BHD. in fact the guy i spoke to spent a couple of hours to write a script there and then to export the 3di files as .obj to export to 3dsmax, something which ive never seen before.
anyway you are going to need a .pff extractor, you might have that already.
the easiest thing after that (based on even i got it to work), was unwrap3d
https://www.unwrap3d.com/u3d/downloads.aspx with the Delta Force 1/2/3/4/5 (3DI) plugin:
the app is free to use, you just cant export as a different filetype, but it will open all the 3di files to view. it will tell you what texture files are associated with that file, but you may just have to look at those in your image app instead.
hope that help