Hi Dale,
As mentioned already, this approach to cost savings can and will very likely be an exercise in futility and frustration. I would start with the reference to rotary trimmers. Well, there are rotary trimmers and then there are good rotary trimmers. For more years than I would want to mention I have been using the RotaTrim trimmers/cutters and have never had any sort of issues with them. I use the 15" and 24" varieties and are definitely not inexpensive. They are however excellent trimmers, made in Sheffield England and will last a lifetime of use, especially the second(current) iteration. My first one was purchased in 1974 and I've often wished it had a cutomter installed. I know it has seen at least a million + cuts during its lifetime and I have never had need to sharpen the blade, since it is self-sharpening with every cut. I still use that original cutter for pieces that don't require exact precision, and it still works as it always has. These are the definition of quality rotary trimmers, although I'm aware there is another brand that I would also recommend if I could recall the name, also very expensive. The bottom line being that if you are really bent on this approach you will likely need the proper tools to do so. The other very important tool would be the D-Roller, since your 3" diameter DIY unit simply is not going to do the job on most papers, especially resin based ones(Luster, Gloss, Semi-Gloss etc). Matte papers are somewhat easier and quicker to flatten, but no guarantee they will not cause destructive head strikes. Again, the D-Roller is not inexpensive, but it does do the job every time.
Hope this helps Dale. Good luck.
Gary