* @param <V2> the type returned by the second future
     * @param <V3> the type returned by the third future
     * @param <U> the type returned by the function
     */
    public interface ClosingFunction3<
        V1 extends @Nullable Object,
        V2 extends @Nullable Object,
        V3 extends @Nullable Object,
        U extends @Nullable Object> {
      /**