I like 3rd level combat magic to be something not terribly common. It's what makes sense in the world to me. Sure, some opponents are going to plan around the possibilty their targets have some sort of magic grenades, but most won't.
I don't think most real world muggers/bandits (current or historical) considered their targets would potentially carry a grenade. I'd consider fireball to be less relatively common and available than grenades.
In a D&D military situation, fireballs would also be less common than grenades (and similar) in the real world.
While many powerful, intelligent, or "always prepared" types would avoid bunching up for a devastating AoE, for most opponents and their leaders you are facing it has never and will never come up, and they aren't going to plan their tactics around it, any more than around dominate spells and similar.
Or in other words, enemies being prepared for the possibilty of fireballs is something I consider highly situational.