Correct way to define C++ namespace methods in .cpp file

Question

Probably a duplicate, but not an easy one to search for...

Given a header like:

namespace ns1
{
 class MyClass
 {
  void method();
 };
}

I've see method() defined in several ways in the .cpp file:

Version 1:

namespace ns1
{
 void MyClass::method()
 {
  ...
 }
}

Version 2:

using namespace ns1;

void MyClass::method()
{
 ...
}

Version 3:

void ns1::MyClass::method()
{
 ...
}

Is there a 'right' way to do it? Are any of these 'wrong' in that they don't all mean the same thing?