Random spawning, but one definite object

Basically you'll need an array of GameObjects to be spawned and an array of Vector3 locations where these objects could be spawned to. Once these are populated you then randomly select an object and and then randomly select a location. Once this is done you instantiate the object at the selected location. Something like this perhaps...

 public GameObject[] MyObjects;
 public Vector3[] MyLocations;

You can then set/populate these in the inspector. Next in your 'Update' method you'll need some sort of timer to trigger the spawning -or- an method that can be called to trigger the spawning. This method will call a function to generate the two random numbers.

 int ObjectID = Random.Range(0, 2); // Returns integer from 0 to 2
 int LocationID = Random.Range(0, 5); // Returns integer from 0 to 4
 

Finally you can call the instantiate method to make it happen...

 GameObject SpawnedObject = (GameObject)Instantiate(MyObjects[ObjectID], MyLocations[LocationID ], Quaternion.identity);

Those are the basics ... hope this helps ;)

UPDATE

Ok ... looks like I made one typo on the randomization code (should have been 3 not 2) but here is a script for you to try ;)

It has both a timed spawn and a triggered spawn (e.g. a GUI button). Note that the Instantiate code is returning the spawned object into a GameObject variable called SpawnedObject so you can alter it after it's spawned (e.g. re-name/re-tag, change scale/rotation/materials, etc.).

 using UnityEngine;
 using System.Collections;
 
 public class Spawner : MonoBehaviour
 {
     public GameObject[] MyObjects;
     public Transform[] MyLocations;
     public float MyTimer = 5f;
 
     private float CurTime;
 
     // Use this for initialization
     void Start () {
         // Initialize the timer value
         CurTime = MyTimer;
     }
     
     // Update is called once per frame
     void Update () {
 
         // Decrement timer value
         CurTime -= Time.deltaTime;
         // If time has run out
         if (CurTime <= 0f)
         {
             // Call the spawner
             SpawnTrigger(); 
             // Reset the timer value
             CurTime = MyTimer;
         }
     }
 
     void OnGUI()
     {
         //If they click the button
         if (GUI.Button(new Rect(Screen.width / 2f - 50f, Screen.height * 0.1f, 100f, 20f), "Spawn Now"))
         {
             // Call the spawner
             SpawnTrigger();
             // Reset the timer value
             CurTime = MyTimer;
         }
     }
 
     void SpawnTrigger()
     {
         int ObjectID = Random.Range(0, 3); // Returns integer from 0 to 2
         int LocationID = Random.Range(0, 5); // Returns integer from 0 to 4
 
         GameObject SpawnedObject = (GameObject)Instantiate(MyObjects[ObjectID], MyLocations[LocationID].position + Vector3.up * 25f, Quaternion.identity);
     }
 }
   

Enjoy ;)