C :

#include <stdio.h>
#include <stdlib.h>
int f(int n)
{
    int i,r=1,m;
    if(n%3==0)
    {
        if(n==0)
        {
            return 0;
        }
        for(i=1;i<=n/3;i++)
        {
            r=r*4;
        }
        return r;
    }
    else if(n%3==1)
    {
        if(n==1)
        {
            return 1;
        }
        else
        {
            r=f(n-1);
            r=r*1+n-1;
            return r;
        }
    }
    else if(n%3==2)
    {
        if(n==2)
        {
            return 2;
        }
        else
        {
            r=f(n-2);
            r=r*2+n;
            return r;
        }
    }
}
int main()
{
    int a,n=0;
    scanf("%d",&a);
    n=f(a);
    printf("%d",n);
    return 0;
}

C++ :

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

Python :

n=int(input())
a=[0 for i in range(n+1)]
for i in range(1,n+1):
    if(i==1):
        a[1]=1
    elif(i==2):
        a[2]=2
    elif(i==3):
        a[3]=4
    else:
        a[i]=a[i-1]+a[i-2]+a[i-3]
print(a[n])