Spawning game Object after a period of time

Hey, i have little experience, i'm using C#. I am trying to spawn a series of Quads (backgrounds) in to a game. I want the first background to spawn when the game is started. Then after X seconds i want the second background to spawn. Then after Y seconds ( from time = 0 )i want the third background to spawn.

I want each background to destroy the previous but haven't attempted this as i cant get then to spawn.

i can get background one to spawn but neither of the other two. Any help will be greatly appreciated.

Here is the code i have so far:

 using UnityEngine;
 using System.Collections;
 
 public class BackGround : MonoBehaviour {
 
     public GameObject background_one;
     public GameObject background_two;
     public GameObject background_three;
     float timer = 0f;
 
    
         
     // Use this for initialization
     void Start () {
         
         Instantiate(background_one, new Vector3(0,0,0), Quaternion.identity);                
     }
 
                 
     // Update is called once per frame
     void update () {
                 
         timer += Time.deltaTime;    
        
         if (timer >= 2)
         {
             Instantiate(background_two, new Vector3(0, 0, 0), Quaternion.identity);
         }
 
         if (timer >= 4)
         {
             Instantiate(background_three, new Vector3(0, 0, 0), Quaternion.identity);
         }
     }
             
     }