When to use get, get_queryset, get_context_data in Django?

They indeed do different things.

get()

This is a top-level method, and there's one for each HTTP verb - get(), post(), patch(), etc. You would override it when you want to do something before a request is processed by the view, or after. But this is only called when a form view is loaded for the first time, not when the form is submitted. Basic example in the documentation. By default it will just render the configured template and return the HTML.

class MyView(TemplateView):
    # ... other methods

    def get(self, *args, **kwargs):
        print('Processing GET request')
        resp = super().get(*args, **kwargs)
        print('Finished processing GET request')
        return resp

get_queryset()

Used by ListViews - it determines the list of objects that you want to display. By default, it will just give you all for the model you specify. By overriding this method you can extend or completely replace this logic. Django documentation on the subject.

class FilteredAuthorView(ListView):
    template_name = 'authors.html'
    model = Author

    def get_queryset(self):
        # original qs
        qs = super().get_queryset() 
        # filter by a variable captured from url, for example
        return qs.filter(name__startswith=self.kwargs['name'])

get_context_data()

This method is used to populate a dictionary to use as the template context. For example, ListViews will populate the result from get_queryset() as author_list in the above example. You will probably be overriding this method most often to add things to display in your templates.

def get_context_data(self, **kwargs):
    data = super().get_context_data(**kwargs)
    data['page_title'] = 'Authors'
    return data

And then in your template, you can reference these variables.

<h1>{{ page_title }}</h1>

<ul>
{% for author in author_list %}
    <li>{{ author.name }}</li>
{% endfor %}
</ul>

Now to answer your main question, the reason you have so many methods is to let you easily stick your custom logic with pin-point accuracy. It not only allows your code to be more readable and modular, but also more testable.

The documentation should explain everything. If still not enough, you may find the sources helpful as well. You'll see how everything is implemented with mixins which are only possible because everything is compartmentalized.

Let's look at the default implementation of ListView's get method:

https://github.com/django/django/blob/92053acbb9160862c3e743a99ed8ccff8d4f8fd6/django/views/generic/list.py#L158

class BaseListView(MultipleObjectMixin, View):
    """
    A base view for displaying a list of objects.
    """
    def get(self, request, *args, **kwargs):
        self.object_list = self.get_queryset()
        allow_empty = self.get_allow_empty()

        if not allow_empty:
            # When pagination is enabled and object_list is a queryset,
            # it's better to do a cheap query than to load the unpaginated
            # queryset in memory.
            if (self.get_paginate_by(self.object_list) is not None
                    and hasattr(self.object_list, 'exists')):
                is_empty = not self.object_list.exists()
            else:
                is_empty = len(self.object_list) == 0
            if is_empty:
                raise Http404(_("Empty list and '%(class_name)s.allow_empty' is False.")
                        % {'class_name': self.__class__.__name__})
        context = self.get_context_data()
        return self.render_to_response(context)

You will notice that get_queryset gets called in the first line. You can simply overwrite that if you just want to return your model's queryset after applying some filtering/ordering etc.

You don't need to overwrite the whole get method for that because you will be missing on all this provided functionality i.e. pagination, 404 checks etc.

get_context_data merges the resulting queryset together with context data like querystring parameters for pagination etc.

What I would recommend would be to check with django's source every once in a while and try to understand it a little bit so that you can recognize the most appropriate method you can overwrite/replace.