Instantiating a prefab as a child of an instantiated prefab

I have a hierarchy that looks like this:

 Instantiated Prefab
       Item1
 
       Item2
 
       Item3

I understand that prefabs can not be nested (placing the 2nd Prefab inside of the first prefab in the project window).

I want my hierarchy to look like this:

 Instantiated Prefab
       Item 1
 
       Item 2
 
       Item 3
         Second Instantiated Prefab

How can I go about achieving this using c#?