C++ :

#include <iostream> 
#include <cstdio>
using namespace std;
int main()
{
	int x;
	cin>>x;
	if(x%3 == 0)
	{
		cout<<"yes"<<endl;
	}
	else
	{
		cout<<"no"<<endl;
	}
	return 0;
}

Pascal :

var
a,b,c:integer;
begin
readln(a);
if a mod 3=0 then writeln('yes')
else writeln('no');
end.