Feed back to StackOverflow Question. https://stackoverflow.com/q/79819532/2423906

App.java

package sed.home.javafxfxmltestingground;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

import java.io.IOException;

/**
 * JavaFX App
 */
public class App extends Application {

    private static Scene scene;

    @Override
    public void start(Stage stage) throws IOException {
        scene = new Scene(loadFXML("primary"), 640, 480);
        stage.setScene(scene);
        stage.show();
    }

    static void setRoot(String fxml) throws IOException {
        scene.setRoot(loadFXML(fxml));
    }

    private static Parent loadFXML(String fxml) throws IOException {
        FXMLLoader fxmlLoader = new FXMLLoader(App.class.getResource(fxml + ".fxml"));
        return fxmlLoader.load();
    }

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

}

primary.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.layout.VBox?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.ComboBox?>
<?import javafx.geometry.Insets?>

<VBox alignment="CENTER" spacing="20.0" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sed.home.javafxfxmltestingground.PrimaryController">
   <padding>
        <Insets bottom="20.0" left="20.0" right="20.0" top="20.0" />
    </padding>
   <children>
      <ComboBox fx:id="cb1" onAction="#OnCB1Selected" />
      <ComboBox fx:id="cb2" onAction="#OnCB2Selected" />
      <ComboBox fx:id="cb3" onAction="#OnCB3Selected" />
   </children>
</VBox>

PrimaryController.java

package sed.home.javafxfxmltestingground;

import java.util.ArrayList;
import javafx.collections.FXCollections;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.scene.control.ComboBox;

public class PrimaryController {
    @FXML
    private ComboBox<String> cb1;
    @FXML
    private ComboBox<String> cb2;
    @FXML
    private ComboBox<String> cb3;

    private final ArrayList<ComboBox<String>> comboBoxes = new ArrayList();

    private ComboBox<String> currentComboBox;

    public void initialize() {
        ArrayList<String> strsList = new ArrayList();
        strsList.add("A");
        strsList.add("B");
        strsList.add("C");
        strsList.add("D");
        strsList.add("E");
        strsList.add("F");
        strsList.add("G");
        strsList.add("H");
        
        cb1.setItems(FXCollections.observableArrayList(new ArrayList(strsList)));
        cb2.setItems(FXCollections.observableArrayList(new ArrayList(strsList)));
        cb3.setItems(FXCollections.observableArrayList(new ArrayList(strsList)));
        
        comboBoxes.add(cb1);
        comboBoxes.add(cb2);
        comboBoxes.add(cb3);
    }
   

    @FXML
    private void OnCB1Selected(ActionEvent actionEvent) {
        currentComboBox = (ComboBox)actionEvent.getSource();
        removeItemFromComboBoxes();       
    }
    
    @FXML
    private void OnCB2Selected(ActionEvent actionEvent) {
        currentComboBox = (ComboBox)actionEvent.getSource();
        removeItemFromComboBoxes();       
    }

    @FXML
    private void OnCB3Selected(ActionEvent actionEvent) {
        currentComboBox = (ComboBox)actionEvent.getSource();
        removeItemFromComboBoxes();       
    }    
    
    private void removeItemFromComboBoxes()
    {
        if(currentComboBox.getValue() != null)
        {
            comboBoxes.forEach(node -> {
                if(node != currentComboBox)
                {
                    node.getItems().remove(currentComboBox.getValue());
                }
                else
                {
                    //Use if needed!
                }
            });
        }
    }
}