Tracking and targeting one of multiple objects

So here's what I have set up currently:

There are three objects, A B and C, all on a plane. Object A will move about the level if you click a location, much like a dungeon crawler or a MOBA, Object B remains in place unless moved in the Editor, and Object C will move towards objects A and B if they are within a radius of 10.

The script I have set up puts all of the enemies in an array based on their tag and then uses that to calculate the distance of each one in the Update function/method (currently using Javascript, wishing I was using C#, too lazy to change now). If it finds something within its radius, it sets that object as the closest available, sets the bool targetFound to true, and then defines the target variable as whatever the closest is for later use.

Here's my problem:

I want the object to function much like the minions of League of Legends, where they will track object A so long as it is present in their line of sight, and then move on to another, namely B, if A leaves the stage. I just don't know how to set this up in the least. Furthermore, my current code only returns null.

Here's my code:

 #pragma strict
 
 var target;
 var targetFound = false;
 
 function Update() {
     Debug.Log(FindClosestEnemy());
     // if(FindClosestEnemy() != null && targetFound == true) {
     //     transform.position = Vector3.MoveTowards(transform.position, FindClosestEnemy().transform.position, 3 * Time.deltaTime);
     // }
 }
 
 // Find the closest available enemy
 function FindClosestEnemy () : GameObject {
     var gos : GameObject[];
     gos = GameObject.FindGameObjectsWithTag("Champion");
     
     var closest : GameObject = null;
     var los : float = 50;
     var position = transform.position;
     
     // Check every instance of the array of enemy characters
     for(var go : GameObject in gos) {
         // Calculate difference in distance between the objects
         var diff = (go.transform.position - position);
         // Equate the actual current distance
         var curDistance = diff.sqrMagnitude;
         // Check if the current distance is less than the line of sight
         if (curDistance < los) {
             // Check if we're already targeting something else
             if(targetFound != false) {
                 // Enemy must be nearby
                 closest = go;
                 // We are now targeting that enemy
                 targetFound = true;
                 // Declare the target
                 target = closest;
                 // We're kinda done here
                 return target;
             // Otherwise, we must already have a target
             } else {
                 return target;
             }
         }
     }
 }