Move the "using" above the #include "Multifile.h" line.

It's not a good idea to put function bodies in a header file because if that header file is included in more than one source file (which is the point of header files, so if you only have one source file you don't need one) then you're going to get duplicate symbols. So just get it all working in one file before splitting it out into headers.

This all works fine in Visual Studio 2005 (main() just calls FUNC() which stops me needing millions of projects for go4e code testing):
#include <iostream>

#define FUNC go4e_16291

#include <string>
using namespace std;

int times3(int number) { return (number * 3); }
string times3(string text) { return (text + text + text); }

void go4e_16291()
cout << "Tripling 9: " << times3(9) << endl;
cout << "Tripling 'game': " << times3("hello") << "\n\n";
Tripling 9: 27
Tripling 'game': hellohellohello