Version 2 is unclear and not easy to understand because you don’t know which namespace MyClass belongs to and it’s just illogical (class function not in the same namespace?)
Version 1 is right because it shows that in the namespace, you are defining the function.
Version 3 is right also because you used the :: scope resolution operator to refer to the MyClass::method () in the namespace ns1. I prefer version 3.
See Namespaces (C++). This is the best way to do this.