Two stones is not enough, you can then end up with two spherical surfaces. With three this isn't possible (imagine two of them are convex, when you rub them together they will grind eachother down and become less convex).
Is it a strict requirement that one plate remains stationary?
Altough, I disagree with the "random fashion". Alternating the pairs AB -> BC -> CA seems more logical to me.
Also, the order being random wouldn't effect the end result.
however, now my imagination is now running, and thinking about how overkill "true" random might be for this application. something "random" like an orbital sander would probably be enough. you're just trying to get away from side-to-side, left-circle, right-circle, up/down patterns. you're doing this to 3 different surfaces, so they would grind out any slight patterns which is the point. seems like a try crypto random would be as "effecient" as my roomba appears not to be.
Maybe some code then, instead: https://www.dwitter.net/d/20446
I'll add that this depends on the materials having similar properties, so when ground together, they are grinding each other. This means whatever residual shape you have in one of them, it won't be transferred completely to the other.
If the materials had very different hardnesses, one of them would dominate over the other when they're ground together.
https://pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/M...