Spawn moving GameObjects with identical gaps!

So I am trying to spawn platforms with identical gaps. I have different three different kind of sizes of these platforms and I want to spawn them with same gap even they are different size.

 public class Spawner : MonoBehaviour
 {
     private GameObject[] platform;
     private bool spawn;
     private GameObject rollingEnemy;
     Vector3 oldSpawnPos;
     GameObject platformClone;
     public float spawnTime;
 
     void Start() 
     {
         platform = new GameObject[3];
         platform[0] = Resources.Load("Level/Platform1") as GameObject;
         platform[1] = Resources.Load("Level/Platform2") as GameObject;
         platform[2] = Resources.Load("Level/Platform3") as GameObject;
         spawn = true;
         spawnTime = 1.5f;
         oldSpawnPos = Vector3.zero;
     }
 
     void Update() 
     {
         // Spawns new platform
         if(spawn)
         {
             StartCoroutine(spawnPlatforms());
         }
     }
 
     IEnumerator spawnPlatforms()
     {
         spawn = false;   
         int randomPlatform = Random.Range(0, 2);
 
         // New platform
         platformClone = Instantiate(platform[randomPlatform], Vector3.zero, Quaternion.identity) as GameObject;
         platformClone.transform.position = new Vector3(oldSpawnPos.x + (platformClone.GetComponent<BoxCollider>().size.x / 2) + 5f, 0f, 0f);
         oldSpawnPos = platformClone.transform.position; 
 
         yield return new WaitForSeconds(spawnTime);
         spawn = true;
     }
 }

Here is also wonderful picure what I'm trying to do :D!

Thanks already for answering! Every answer is closer to the solving the problem!