Need help with randomizing starting location of terrains

Hello everyone! I've just starting programming and I've hit a roadblock. I have written a code which instantiates 8 terrains at 8 starting positions. Whenever the game is launched, these 8 terrains shuffle between each other. Here is the code I used:

 public class StartingPosition : MonoBehaviour {
 
     public List<GameObject> Terrains; //create array for terrains
     
     public Transform[] TerrainsPositions; //for the predetermined position of the terrains
     
     // Use this for initialization
     void Start () {
     
     foreach (Transform CurrentTerrainPosition in TerrainsPositions) 
     {
         int randomTerrain = UnityEngine.Random.Range(0,Terrains.Count);
             
         Instantiate(Terrains[randomTerrain], CurrentTerrainPosition.transform.position, CurrentTerrainPosition.transform.rotation);
         
         Terrains.RemoveAt(randomTerrain);
     }

What I want to do is this: I want to have 8 starting locations with an array of 3 terrains each. Of these 3 terrains, there will always be a default one which will instantiate under normal circumstances. However, I want the code to pick 3 of the 8 starting locations at random and for those starting locations randomly instantiate 1 of the 3 terrain options I mentioned. This way, whenever the game is launched, the map will not be the same and there will be a lot of variety and incentive for the player to explore.

I think I have an idea of how to use the code I have to create the array of the 3 terrain options for each of the 8 starting positions, but I have no idea how to randomly apply these changes to 3 of the 8 terrains only and how to chose the default one if the starting positions isn't part of the 3 which were chosen by the code.

Any help will be appreciated. Thanks in advance!