How to create serval button after click a button

I want to create a button which is when player clicked,three button will appear.Player can choose which level to play when they click those three button.

That's easy:

if (GUILayout.Button("click to show more buttons"))
{
  if (GUILayout.Button("Button A")) 
    DoSomething();
  if (GUILayout.Button("Button B")) 
    DoSomethingElse();
  if (GUILayout.Button("Button C")) 
    DoSomethingCompletelyDifferent();
}

var showButtons : boolean = false;

if (GUILayout.Button("click to show more buttons"))
{
  showButtons = true;
}

if (showButtons)
{
  if (GUILayout.Button("Button A")) 
    DoSomething();
  if (GUILayout.Button("Button B")) 
    DoSomethingElse();
  if (GUILayout.Button("Button C")) 
    DoSomethingCompletelyDifferent();
}

try this tweak of Jake L's suggestion.