Well, it's pretty obvious not all people agree with the OP's ideals.
I would seriously hate to be limited to three powders. Ten maybe.
Every firearm has its own personality. For instance, 22 bores like different powders than 308 bores and so on.
Just because a certain powder will give acceptable performance, doesn't mean other powders won't give better performance all around in a particular firearm.
I will admit, I have some powders that are the base I work from. Then again, my interests are in intermediate size cases and small cases. Recently, I have developed an urge for the 45-70 cartridge. It is completely different than most of what I shoot now, for the most part.
I have a couple of 45-70s including an original US 1873 Trapdoor Cadet rifle and a Siamese Mauser conversion. I don't like shooting the 1873 for a couple of reasons, one, if something breaks, original parts are hard to come by and it should be fed black powder with cast bullets. The Siamese on the other hand will take very close to a compressed load of IMR3031 with no problems, other than the pain tolerance of the shooter. I have a 1874 Sharps on the way and that should fall somewhere in the middle. Again, a different powder will be required.
I have a couple of different options for the 223, 22 Hornet, 308 Win etc. Yes, I have one powder that will work in all of them acceptably but each of them have powders that work exceptionally well. Why limit myself???
If storage or money is the issue, fine. I did similar things when I was younger and limited in both areas. You have to live within your limitations until you no longer have to.
Every one of us has their personal reasons to use the powders we do. Sometimes it's because we're so familiar with a certain powder/primer combo it is easy for us to figure out the parameters they will operate in well. In many cases, like some of the milsurp cases we are more familiar with today, like the 303 Brit, 7.62x54, 7.62x51 etc. One powder would easily cover all of them, not just acceptably but allow them to perform to their full potential.
One powder I really like, comes from the cases of the cheap 8x63 Swede machine gun ammo Marstar dumped onto the market a few years back. It works very well in the 8x57 Mauser case as well as the 30-06 case with heavy bullets. I will be sad when it's all gone. I have a rifle chambered for the case but the recoil is more than I can handle these days.
No, three powders would make me feel really restricted.