So, the player have 3 bullets?, and will get 3 more when they collide with the enemy:
Create a variable on your bullet called something like GiveAmmo. It's number should be 3.
On left click if GiveAmmo is Equal to 1,2,3, Shoot bullet
When bullet shot, Subtract 1 from GiveAmmo
When bullet collides with enemy, Set GiveAmmo to 3.
like that ?