Como eu faço para alternar entre os nodes dentro do StackPane no JavaFX?

VBox vbox = new VBox();

Button btnCard1 = new Button("CARD 1");
Button btnCard2 = new Button("CARD 2");
Button btnCard3 = new Button("CARD 3");

Pane sp = new StackPane();
Group group1 = new Group(new Label("LAYOUT CARD 1"));
Group group2 = new Group(new Label("LAYOUT CARD 2"));
Group group3 = new Group(new Label("LAYOUT CARD 3"));

btnCard1.setOnAction(new EventHandler<ActionEvent>() {
    public void handle(ActionEvent t) {
        sp.getChildren().clear();   // <--- seu StackPane !
        sp.getChildren().add(group1); // <-- adiciona Group ao StackPane !
    }
});

// etc, etc...

vbox.setPrefSize(300,300);
vbox.getChildren().addAll(btnCard1,btnCard3,btnCard3,sp);
 
stage.setScene(new Scene(vbox));