Style question for packaging methods into a namespace

Ruby
1

From what I can see, the following three ways are equivalent, when we
want to group methods into a namespace such that they can be called as,
i.e., NS.foo or NS.bar (or, equivalently, using a Module to get the
effect of a singleton class):

(1) Explicit self

module NS
  def self.foo; ....; end
  def self.bar; ....; end
end

(2) Implicit self

module NS
  extend self;
  def foo; ....; end
  def bar; ....; end
end

(3) module_function

module NS
  def foo; ....; end
  def bar; ....; end
  module_function :foo, :bar
end

So far, I have nearly always used variant (2), because it seemed to me
the easiest. Any opinions on this, i.e. whether maybe (1) or (3) would
be a better way to do it?

Ronald