I've seen a guy convert a five window to three. As the other's said, if you can cut and weld, there are pre-made panels/skins available and, it's your truck, do what makes you happy.
If it were me and my cab were a three window in the same condition my current cab is in, I would look for a five window cab first that didn't have any rust or other major defects, as mine has a bit of boot rot. They can be had pretty cheap and, if you can find one locally, will take less effort than welding in an inner and outer upper rear cab section or corners. Cutting and welding would be my plan B. ...but thats just my take on it.
