Imo I think we might see each of the original logia admirals specialize in a different form of armament
Aokiji- the best at hardening. Possibly being able to use full body, an advanced technique similar to his own version of edged mochi, and the flame variety where he can perfectly infuse it with his body. All off which can boost his striking power making him the hardest hitter of the 3
Kizaru- the best at barriers. It’s what he taught shutenmaru with him even hyping the fact his defense is better. Probably making him the toughest of the 3 to hurt when serious
Akainu- we don’t know anything about his armament but him specializing in the last version would be fitting as the strongest of the bunch. Him being the best at that making him even more of the most lethal of the 3
As for their observation haki differences idk