How to randomly replace the main platform with additional ones?

Hello. I am new to programming. Difficulties arose in creating multiple platforms that would randomly replace the main platform. There are 1 main and 4 additional platforms in total. The problem with this code is that it doesn't work correctly during platform generation. I would be glad for any help. using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class Spawner : MonoBehaviour
 {
     public float minX, maxX, YrangeMin, YrangeMax, cameraDistance;
     public Transform platformPrefab, movePlatformPrefab, downPlatformPrefab;
     public float percentToSpawnStandartPlatform, percentToSpawnDownPlatform;
 
     private Transform cam;
     private float lastSpawnY;
     private float rangeIncreaser;
     
 
     void Start()
     {
         cam = Camera.main.transform;
         lastSpawnY = 0;
 
     }
 
 
     void Update()
     {
         if (lastSpawnY < 250)
         {
             rangeIncreaser = Mathf.Floor(lastSpawnY / 50);
             
         }
         
 
         if (cam.position.y + cameraDistance > lastSpawnY)
         {
             Transform lawnplatform;
 
             if (Random.value < percentToSpawnStandartPlatform)
                 lawnplatform = Instantiate(platformPrefab);
          
             else
                  lawnplatform = Instantiate(movePlatformPrefab);
 
             if (Random.value < percentToSpawnDownPlatform)
                 lawnplatform = Instantiate(platformPrefab);
          
             else
                  lawnplatform = Instantiate(downPlatformPrefab);
 
 
 
 
 
             lawnplatform.position = new Vector3(Random.Range(minX, maxX), lastSpawnY + Random.Range(YrangeMin + (rangeIncreaser * 0.9f), YrangeMax + (rangeIncreaser * 1.1f)), 0);
 
             lastSpawnY = lawnplatform.position.y;
 
         }
         
     }
 
     
 }