Spawn GameObjects without overlap

Hi! I'm trying to spawn some gameobjects in a chosen area without overlap among them. I save each object position (plus the collider's radius) in 3 lists (one per axis) so the position can not be taken again, but it's not working. This is my code:

 [SerializeField] GameObject nodePrefab;
 [SerializeField] int numberOfObjects;
 [SerializeField] float xValue;
 [SerializeField] float yValue;
 [SerializeField] float zValue;
 private Vector3 spawnPosition;
 private float radiusValue;
 List<float> xPositionList = new List<float>();
 List<float> yPositionList = new List<float>();
 List<float> zPositionList = new List<float>();

 private void Awake()
 {
     radiusValue = nodePrefab.GetComponent<SphereCollider>().radius + 0.15f;
 }

 private void Start () {
     for (int i = 0; i < numberOfObjects; i++)
     {
         float xPos = Random.Range(-xValue, xValue);
         float yPos = Random.Range(-yValue, yValue);
         float zPos = Random.Range(-zValue, zValue);
         while (xPositionList.Contains(xPos)) xPos = Random.Range(-xValue, xValue);
         while (yPositionList.Contains(yPos)) yPos = Random.Range(-yValue, yValue);
         while (zPositionList.Contains(zPos)) zPos = Random.Range(-zValue, zValue);
         spawnPosition = new Vector3(xPos, yPos, zPos);
         Instantiate(nodePrefab, spawnPosition, Quaternion.identity, transform);
         xPositionList.Add(xPos + radiusValue);
         yPositionList.Add(yPos + radiusValue);
         zPositionList.Add(zPos + radiusValue);
         xPositionList.Add(xPos - radiusValue);
         yPositionList.Add(yPos - radiusValue);
         zPositionList.Add(zPos - radiusValue);
     }
 }