Government training is just next level. I think it really does make sense to say that Issho and Aramaki are below C3 simply because they didn't get the full training those three received from Garp and Sengoku.
Not just Garp and Sengoku lol, they were active duty for decades and don’t have time to train fodder. Government military training is going to be the best combat training you can get in OP.
When you have a giant world spanning organization with the largest military and best funding of any organization out there, they are going to possess intricate knowledge on things like haki, Devil fruits, martial arts etc. and are going to have perfected training techniques to teach this stuff to all their top fighters.
Issho and Aramaki likely received formal military training elsewhere, you don’t accidentally forge a black blade/achieve the top CoO in the verse, they had to learn the fundamentals of this stuff somewhere and probably mastered it on their own.