Hi. The following is one way you could do this:
Make sure the 4th sphere has its mesh renderer disabled in the inspector so it is invisible when the scene is played.
Create a global integer.
Create an FSM with a
Mouse Pick Event on each sphere that fires off an event to add 1 to this global integer when it is clicked.
Have an FSM on the 4th sphere that watches the global integer (use
Int Compare action) and enables the mesh renderer on itself when the global integer reaches 3.
You can access the mesh renderer settings on the 4th sphere by using the
Set Property action, or by dragging the object onto the action window.
Hope that makes sense, happy to clarify anything if needed
Something like this on three spheres:
And this on the fourth sphere: