First go

ArtistSongGenre.rb

class Artist
  attr_accessor :name, :songs, :genres

  @@artists = []

  def initialize
    @@artists << self
    @songs = []
    @genres = []
  end

  def self.reset_artists
    @@artists.clear
  end

  def self.count
    @@artists.count
  end

  def self.all
    @@artists
  end

  def add_song(song)
    songs << song
    if song.genre
      genres << song.genre
      song.genre.artists << self
    end
  end

  def songs_count
    songs.count
  end

end

class Song
  attr_accessor :genre

  def genre=(genre)
    genre.songs << self
    @genre = genre
  end
end

class Genre
  require 'set'
  attr_accessor :name, :songs, :artists

  @@genres = []

  def initialize
    @@genres << self
    @songs = []
    @artists = Set.new
  end

  def self.reset_genres
    @@genres.clear
  end

  def self.all
    @@genres
  end
end