JavaFX: Splitpane columns not equal

Question

I want to build a javafx application, that consists of 3 big columns about the whole scene. The columns should be resizable, so i used splitpane. I added 3 borderpanes to the splitpane, every with the same preferred width (The scene is overall 3840 width, so each column is 1280 width). But when i start the application, the middle borderpane is smaller then the two other. It is not scaled on 1280 width.

So how can i manage that the splitpane is not scaling the borderpanes and use the preferred width?

Update: I have found a good solution, that makes the solution from user3249346 dynamic. After adding or deleting a column you can call this:

ObservableList<SplitPane.Divider> dividers = splitPane.getDividers();
for (int i = 0; i < dividers.size(); i++) {
    dividers.get(i).setPosition((i + 1.0) / 3);
}

Answer 1

I tried this and this works when i maximize and restore the screen with three columns of Splitpane. Important is the Platform.runLater

import javafx.application.Application;
import javafx.application.Platform;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.SplitPane;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class SplitPaneTest  extends Application {
@Override public void start(Stage stage) {
     final SplitPane sp = new SplitPane();
     final StackPane sp1 = new StackPane();
     Button addColumn = new Button("Add Column");
       // Added the below function to set the size dynamically when column is added.
     addColumn.setOnAction(new EventHandler<ActionEvent>() {
        @Override
        public void handle(ActionEvent arg0) {
            StackPane temp = new StackPane();
            temp.getChildren().add(new Button("New Column"));
            sp.getItems().add(temp);
            int divCount = sp.getDividerPositions().length;
            double equalSize = 1.0/(divCount+1);
            double divPosValues[] = new double[divCount];
            for(int count=1; count<divCount+1; count++) {
                divPosValues[count-1] = equalSize*count;
            }
            sp.setDividerPositions(divPosValues);
        }
    });

     sp1.getChildren().add(addColumn);
     final StackPane sp2 = new StackPane();
     sp2.getChildren().add(new Button("New Button"));
     final StackPane sp3 = new StackPane();
     sp3.getChildren().add(new Button("New Button"));
     sp.getItems().addAll(sp1, sp2, sp3);
     sp.setDividerPositions(0.33f, 0.63f, 0.93f);

    Scene scene = new Scene(sp, 400, 400);
    stage.setScene(scene);

    Platform.runLater(new Runnable() {
        @Override
        public void run() {
            sp.setDividerPositions(0.3f, 0.6f, 0.9f);
        }
      });
    stage.show();
}

public static void main(String[] args) {
    Application.launch(args);
}
}