In terms of specs, both the Redmi 6A and the Redmi 6 pack in the same 5.45-inch HD+ display, while the Redmi 6 Pro packs in a larger 5.84-inch Full HD+ display. While Redmi 6 offers more RAM and storage than the 6A, they are more or less similar otherwise.
As for the Redmi 6 Pro, it also offers a faster processor to promise better performance overall. It also has dual cameras that promise to offer better photos than the other two devices. Finally, it also has a bigger 4,000 mAh battery which is definitely a big advantage.