How to prevent 3D object spawn overlapping?

I'm on my phone so I can't write code for you. But I can tell you what to do...

Use Physics.OverlapSphere() to check the spawn position before spawning. This method will collect all colliders in the specified area and return them all in an array. Just go through the array in a loop to look for items that you don't want to spawn on top of... Then if you find an item you don't want to spawn on top of, then pick a new random spawn location and repeat the process. (loop)

EDIT:

Try this. You're going to have to play around with the obstacleCheckRadius variable. It determines how big our check area is.

One warning about my code

My code will not spawn an obstacle after 10 attempts. Therefore even if you want 10 obstacles, you might only get 9 if there's not enough space to spawn the last one. I did this to prevent an infinite loop from happening, which would crash your Unity and game.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ObstaclePlacer : MonoBehaviour
 {
     public int ObsToPlace = 10;
     public GameObject[] Obstacles = new GameObject[0];
     GameObject Obstacle;
 
     public float obstacleCheckRadius = 3f;
     public int maxSpawnAttemptsPerObstacle = 10;
 
     void Awake()
     {
 
         for (int i = 0; i < ObsToPlace; i++)
         {
             Obstacle = Obstacles[Random.Range(0, Obstacles.Length)];
 
             // Create a position variable
             Vector3 position = Vector3.zero; 
 
             // whether or not we can spawn in this position
             bool validPosition = false;
 
             // How many times we've attempted to spawn this obstacle
             int spawnAttempts = 0;
 
             // While we don't have a valid position 
             //  and we haven't tried spawning this obstable too many times
             while(!validPosition && spawnAttempts < maxSpawnAttemptsPerObstacle)
             {
                 // Increase our spawn attempts
                 spawnAttempts++;
 
                 // Pick a random position
                 position = new Vector3(Random.Range(-50.0f, 50.0f), 0, Random.Range(-50.0f, 50.0f));
 
                 // This position is valid until proven invalid
                 validPosition = true;
 
                 // Collect all colliders within our Obstacle Check Radius
                 Collider[] colliders = Physics.OverlapSphere(position, obstacleCheckRadius);
 
                 // Go through each collider collected
                 foreach(Collider col in colliders)
                 {
                     // If this collider is tagged "Obstacle"
                     if(col.tag == "Obstacle")
                     {
                         // Then this position is not a valid spawn position
                         validPosition = false;
                     }
                 }
             }
 
             // If we exited the loop with a valid position
             if(validPosition)
             {
                 // Spawn the obstacle here
                 Instantiate(Obstacle, position + Obstacle.transform.position, Quaternion.identity);
             }
         }
     }
 }