Try this(it was not tested), assign the Transforms in the inspector.
import System.Collections.Generic; // To use list
#pragma strict
var spawnPositions : List.<Transform> = new List.<Transform>(); // Add as many as you want in the inspector
var spawnIndex : int = 0;
function Update ()
{
// If not invoking, invoke the spawn method.
if (!IsInvoking("spawn"))
Invoke("spawn", 3); // Invoke every 3 seconds if not invoking already
}
function spawn ()
{
this.transform.position = spawnPositions[spawnIndex].position; // Set this gameobjects transform to the spawnIndex of spawnPositions
spawnIndex++; // Increment the currentIndex
// The index is 0 based when accessing items in the list, we need to compare the count minus 1 to see if we need to reset the spawnIndex.
if (spawnIndex > spawnPositions.Count - 1)
spawnIndex = 0;
}