How to make a game object appear, when another game object is clicked?

A few ways to do this, I'll try to give you the simplest. Create your 3 planes in the editor. Make the plane you are going to click the parent of the 2 planes that will be invisible at first. Disable the mesh colliders for the invisibles. Then attach a script to your "main" plane, and use an [OnMouseDown][1] function. In this function, since your other two planes are the children, use [GetComponentsInChildren][2] to find the [MeshRenderer][3] components in each. On startup, set these to have 0 alpha in their color. Here's a C# example. Check the links I posted for more help.

 using UnityEngine;
 using System.Collections;
 
 public class EXAMPLE : MonoBehaviour
 {
     MeshRenderer[] planes;
     Color color;
 
     void Start()
     {
         color = Color.grey;
         color.a = 0;
         planes = GetComponentsInChildren<MeshRenderer>();
         foreach(MeshRenderer plane in planes)
         {
             plane.renderer.material.color = color;
         }
     }
 
     void OnMouseDown()
     {
         StartCoroutine(FadeInChildren());
     }
 
     IEnumerator FadeInChildren()
     {
         while(true)
         {
             color.a+= 0.01f;
             if(color.a >= 1.0f)
             {
                 StopAllCoroutines();
             }
             foreach(MeshRenderer plane in planes)
             {
                 plane.renderer.material.color = color;
             }
             print (color.a);
             yield return new WaitForEndOfFrame();
         }
     }
 }