Flurry answer: I'm pretty sure excellencies apply to individual rolls, so it would go 7/4/3. Luckily Infinite Melee Mastery, or similar, will make applying two dice to the entire flurry free once you can get it.
Simple stunt answer: Stunts have to apply to entire flurries, otherwise the game breaks. This is because the ability to flurry every individual attack leads to the ability to gain enough motes to pay for an action long perfect every action, and that means nobody can ever die.
Complex stunt answer: Once you've said that the attacker can stunt each attack separately, then it stands to reason the defender can stunt each defense separately. Assuming your following the core books definition of 2 dot stunts as merely bringing the environment into play, most players can routinely get them and pay for their perfect defenses. If you don't follow the core book on stunts to prevent this, then the next question is if you use unexpected attacks. If you do, then players need to be able to combo their defense or a surprise attack will kill them. This means that any attacker with sufficient stealth and the endurance to survive until your players' starting willpower runs out will kill them, which generally isn't considered fun. If you don't use the stealth rules, then, because of overdrive, nobody needs to combo every action once combat gets going. If you still allow your players to get one dot stunts routinely they can survive forever as long as they can meet the condition of the cost lowering charms on their perfects, which in at least some cases is pretty easy. Further, once you start restricting your players stunts charms like Tragic love amusement, which give stunt bonuses, become much higher valued.