I, as well as every other Metroid Prime fan, can guarantee you that you have been wrong all along. Corruption's normal mode is an easy difficulty setting that didn't exist in Metroid Prime and Echoes. Corruption's veteran mode is identical in difficulty to normal in Metroid Prime and Echoes. Corruption's hyper mode doubles the amount of damage you take and the amount of enemy health compared to veteran, just like Metroid Prime's and Echoes' hard mode does compared to the normal difficulty found in these two games.
So wait.... Is the hardest difficulty of MP 1/ MP 2 comparable to the hardest difficulty level for MP 3?
Yes, it is. And the the difficulty level below that is also comparable across all games.
The only difference is that Corruption had a third, easier setting that the first two games didn't have.