Getting singleton instance inside it's methods

I've got simple singleton class:

class Singleton
{
    public:
        static Singleton& getInstance()
        {
            static Singleton instance;
            return instance;
        }
        // private constructor, etc...
}

The question is how should I access class instance from it's member functions? I've seen variuos code snippets and now I'm curious if they have any significant differences?

1.

void Singleton::something(){
    Singleton &self = getInstance();
    self.doSomething();
}

2.

void Singleton::something(){
    this->doSomething();
}

3.

void Singleton::something(){
    doSomething();
}