sum3() Data Structure Java

Design a data structure that can efficiently store and check if the sum of any three successfully added elements is equal to a given sum.

Given code:

public class Sum3 {
    /**
     * Adds/appends list of integers at the end of internal list.
     */
    public void addLast(int[] list) {
        throw new UnsupportedOperationException("Waiting to be implemented.");
    }

    /**
     * Returns boolean representing if any three consecutive integers in the
     * internal list have given sum.
     */
    public boolean containsSum3(int sum) {
        throw new UnsupportedOperationException("Waiting to be implemented.");
    }

    public static void main(String[] args) {
        Sum3 s = new Sum3();

        s.addLast(new int[] { 1, 2, 3 });

        System.out.println(s.containsSum3(6));
        System.out.println(s.containsSum3(9));

        s.addLast(new int[] { 4 });

        System.out.println(s.containsSum3(9));
    }
}