Instantiate stop when all of them have destroyed

hallo guys, i have a problem here. i use instantiate for cloning the enemy at random place. the player can shoot the enemy and it will be destroyed. enemy will spawning every 3 second. but if the player shoot all the enemy before next 3 second, enemy will not spawning again. what happen to my enemy? i want to make them keep spawning every 3 second. please help me. anyone can answer what happen to my game?

Yo need to Instatiate a prefab, not a scene gameobject, that is: you have to reference an “object” in your project window, not one in the scene.

Anyway, try to learn about pooling, take a look here for example : Keeping track of items in a pool? - Unity Answers