Create an instance variable shotCounter.
After each shot add 1 to shotCounter.
If shotCounter<3, start short timer to fire another shot.
If shotCounter=3, start a longer ("cooldown") timer.
On "cooldown" timer reset shotCounter to 0 and you can shoot again.
Note that it doesn't take into account LoS.
You might want to add another instance variable - EnemyUID to the turret.
When an enemy is in LoS, save enemy.UID to this variable.
Then fire 3 bullets at that enemy (every time picking it by EnemyUID).
After the "cooldown" timer check LoS again and repeat.