public int countWays(int n) {
    if (n < 0)
        return 0;
    else if (n == 0)
        return 1;
    else {
        return countWays(n-1) + countWays(n-2) + countWays(n-3);
    }
}

countWays(4)

(3,1), (1,3)
(2,1,1), (1,2,1), (1,1,2)
(1,1,1,1)