如何在单击按钮的同时在其他屏幕内打开Java fxml屏幕

I have three screens in fxml .first screen is login screen.After authentication ,on clicking on login button.user enters in second screen.I have done it.Now on screen 2, there is a button called authentication.on clicking this button,third screen is opened inside second screen.How can i achieve it i.e how can i open 3rd screen inside 2nd screen on click of button.plz suggest some way to do this in javafx.I have three screens in fxml .first screen is l