Need assistance with destroying Gameobjects

This below is untested code, but i hope it gives you an idea of how to handle the problem. Instead of a coroutine i just used a timer to simplify things. The timer will run and each time the timer runs out we create a platform. When creating a platform we save a reference to it, so we are able to destroy it later. At the moment I used an arbitrary amount of maximum platforms. I hope the comments will help you out with anything that isn't clear. The most important part of this, is to save a reference to your created objects (in an array or list...), so you can access them later.

 public Transform platformPrefab;
 public Vector3 nextTileSpawn;
 
 //A list to hold all the instantiated platforms.
 public List<GameObject> activePlatforms = new List<GameObject>();
 //How many platforms can be active at once?
 public int maxPlatforms = 3;
 
 //Timer
 public float timerValue = 0.0f;
 public float timerMax = 0.5f;
 
 public void Start()
 {
     //Some conditions here...
     gameRunning = true;
 }
 
 
 public void Update()
 {
     //Increment the timer
     timerValue += Time.deltaTime;
     
     //Has the timer expired?
     if(timerValue >= timerMax)
     {
         //Reset the timer
         timerValue = 0;
         CreatePlatform();
     }
 }
 
 public void CreatePlatform()
 {
     //First create a new platform and add it to the list.
     activePlatforms.Add(Instantiate(platformPrefab, nextTileSpawn, Quaternion.identity));
     //Increment the position for the next tile.
     nextTileSpawn.z += 4;
     //Check how many tiles are active now, if more than allowed, remove the oldest one.
     if(activePlatforms.Count > maxPlatforms)
     {
         //To many platforms, remove the first one.
         GameObject p = activePlatforms[0];
         //Remove from the list.
         activePlatforms.RemoveAt(0);
         //Destroy the object.
         Destroy(p);
     }
     
 }