Launch projectile

well im working on a pretty easy game, im gunna have it so you are on the bottom using the left and right arrow keys, and when you press space bar it shoots somthing up…i used this code on the projectile:[AS]if (key.isDown(key.SPACE)) {
this._y -= 8;
this._y = 350;
this._x = _root.guy._x;
} [/AS]
but it wont launch, also i want it so you can only launch three at a time…how would i do that?..(i attached the file)