Interesting groups found in the Rider-Waite deck:
The three Wise Men, the Three Magi: !, V, IX (Mercury, Taurus, Virgo)
The three Arch Angels: XIV, VI, XX (Sagittarius, Gemini, Fire)
The three heavenly bodies: XIX, XVII, XVIII (Sol, Aquarius, Pisces)
The three calamnities: XIII, XVI, XV (Scorpio, Mars, Capricorn)
The three authorities: IV, VII, XI (Aries, Cancer, Libra)
The three Ladies: II, III, VIII (Luna, Venus, Leo) (Star could be added but Star is in separate group)
The four angels: X, XXI (Jupiter, Saturn/Earth)
The two poles: 0, XII (Air, Water)