Sounds like it's one of those old .3ds models. They're often more hassle to deal with than they're worth. I'm going to guess the model contains all triangulated faces? It's going to be a real problem to create seams for new UVs until you convert the triangulated faces to quads. Also, you may need to do a distance weld on the objects because chances are the vertices aren't welded together. You'll notice this if you select an object and go into edit mode, hover over some faces and press L (select linked). If you're only getting individual pieces of the object then it's not welded.

One fast way to unwrap the model with relatively useable UVs is just to select the parts of the object you need unwrapped, and use Smart UV Project. You won't need to create seams, because it will unwrap by angle. Problem is, this could result in a ton of small pieces of UV islands if the object's vertices aren't welded together and converted to quads.

______________________________________

My Store
My Free Models
My Video Tutorials
My CG Animations
Instagram: @luxxeon3d
Facebook: https://www.facebook.com/luxxeon