gistfile1.rb

resources :a do
  resources :b do
    resources :c
  end
end

Controller c
before_action :set_parents
...
def set_parents
  @parents = []
  @a = A.find( params[:a_id] )
  @parents << @a
  @b = B.find( params[:b_id] )
  @parents << @b
end

c/views/new.html.erb

<%= link_to 'Back', url_for( @parents ) %>  results in a link to /a/1/b/2/c