C :

#include <stdio.h>
#include <stdlib.h>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */
int f(int n){
	if(n==1){
		return 1; 
	}
	if(n==2){
		return 2;
	}
	if(n==3){
		return 4;
	}
	if(n>3){
		return f(n-1)+f(n-2)+f(n-3);
	}
}
int main(int argc, char *argv[]) {
	int n;
	scanf("%d",&n);
		printf("%d\n",f(n));
	
	return 0;
}

C++ :

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
using namespace std;
int main ()
{
 int i,m,n,j;
 long int a[1100];
 cin>>n;
 memset(a,0,sizeof(a));
 a[1]=1;a[2]=2;a[3]=4;
 for (i=4;i<=n;++i)
  a[i]=a[i-3]+a[i-2]+a[i-1];
 cout<<a[n];
 return 0;
}

Pascal :

var f:array[1..1000] of longint;
    n,i:longint;
begin
  readln(n);
  f[1]:=1; f[2]:=2; f[3]:=4;
  for i:=4 to n do
  f[i]:=f[i-1]+f[i-2]+f[i-3];
  write(f[n]);
end.