How to lerp an object's scale?

First Lerp() takes two Vector3 parameters and a fraction parameter. You are passing three floats. Assuming you are doing this to size over time, 'Explosion' would need to get called every frame. I doubt you have it structured that way. You can structure Explosion as a coroutine to make this work. Here is a bit of demo code. Create a new scene, Create a cube, add this code to the cube. While running the up and down arrow keys will make the cube bigger or smaller.

 #pragma strict 
 
 public var scaleBy : float = 1.5;
 public var speed : float = 1.0;
 private var scale : Vector3 = Vector3(1,1,1);
 private var scaling = false;
 private var trans : Transform;
 
 function Start() {
     trans = transform;
 }

 function Update() {
     
     if (!scaling) {
         if (Input.GetKeyDown(KeyCode.UpArrow)) {
             Explosion(scaleBy, speed);
         } 
         else if (Input.GetKeyDown(KeyCode.DownArrow)) {
             Explosion(1.0 / scaleBy, speed);
         }
     }
 }
 
 function Explosion(factor : float, speed : float) {
     scaling = true;
     scale *= factor;
     while (trans.localScale != scale) {
         trans.localScale = Vector3.MoveTowards(trans.localScale, scale, Time.deltaTime * speed);
         yield;
     }
     scaling = false;
 }

The 'scaling' boolean variable is used to lock out input while the cube is scaling. You may not need it for an explosion use.

Simply use Explosion.transform.localScale = Mathf.Lerp(finalScale,finalScale,finalScale);@BZS