Hi.

Yes, "Sphere-Blue" "Sphere-Green" and "Sphere-Red" are the names of the objects I have in the scene, though I'm not sure about the renderer? I haven't attached any renderers, I'm not even sure what that means.

Add this script:

#pragma strict

// Define a button setting data structure
public class ButtonSwitch
{
    public var name      : String; // Name of GUI button
    public var renderer       : Renderer; // Reference to actual 3D object
    public var areaOnScreen    : Rect; // Rectangle to place GUI button
}

public var buttonAction : ButtonSwitch[]; // An array of all button settings

function OnGUI () 
{
    var buttonSwitchedIndex : int = -1;

    // Process all button settings
    for(var b=0; b<buttonAction.Length; b++)
    {
            // Draw button with current settings. Check if a button with current settings was click. 
       if ( GUI.Button(buttonAction[b].areaOnScreen, buttonAction[b].name) )
       {
                    // Show 3D object
         buttonAction[b].renderer.enabled = true;
                    // Remember the place in array of the pressed button
         buttonSwitchedIndex = b;
       }
    }

    // Shutdown the rest of the buttons
    if ( buttonSwitchedIndex > -1 )
    {
       for(b=0; b<buttonAction.Length; b++)
       {
                    // Unless this is the 3D button we showed, hide button
         if ( buttonSwitchedIndex != b )
          buttonAction[b].renderer.enabled  = false;
       }
    }
}

1. Attach it to a new game object.
2. You have a list of "Button Action" - set it to "3" as you have 3 button.
3. Set the name of each of the buttons as you wish to appear on the button label.
4. Into the "renderer" field - drag the obrect corresponding to that button, e.g. to the "Blue" drag the object "Sphere-Blue".
5. "Area On Screen" is the rect you set to place the GUI button.

Now, clicking on the gui button should do the trick.

Renderer is the component that draws a 3D object in your game.