#include<iostream>
using namespace std;
int main(){
    unsigned long long n, a = 1, b = 2, c = 4, d;
    cin >> n;
    if(n == 1 || n == 2)cout << n;
    else{
        for(int i = 3; i < n; i++){
            d = a + b + c;
            a = b; b = c; c = d;
        }
        cout << c;
    }
    return 0;
}

其实这个数列有通项公式，但明显递推更适合计算机

#include<bits/stdc++.h>
using namespace std;
int ways(int x);
int main()
{
    int x;
    cin>>x;
    cout<<ways(x)<<endl;
    return 0;
}

int ways(int x)
{
    if(x==1)
        return 1;
    else
    {
        if(x==2)
            return 2;
        else
        {
            if(x==3)
                return 4;
            else
                return ways(x-1)+ways(x-2)+ways(x-3);
        }
    }
}