The Maximum of 3 Rule states that between your Main Deck, Extra Deck and Side Deck, you may only have three copies of one card of the same name. Here are some examples:

Example 1

The cards "Harpie Lady", "Cyber Harpie Lady", "Harpie Lady 1", "Harpie Lady 2", and "Harpie Lady 3" all state that their name is also "Harpie Lady", therefore they all share the same card name "Harpie Lady", so you can only have a maximum of three between these card names. For example:

• Three copies of "Harpie Lady", no copies of the others.
• One copy of "Harpie Lady 1", one copy of "Harpie Lady 2", one copy of "Harpie Lady 3", no copies of either "Harpie Lady" or "Cyber Harpie Lady".
• Two copies of "Harpie Lady 2", one copy of "Harpie Lady 3", no copies of either "Harpie Lady 1", "Harpie Lady", or "Cyber Harpie Lady".

"Harpie Queen", on the other hand, only has its name treated as "Harpie Lady" while it is face-up on the field, or in the Graveyard, but not in the Main Deck, so you can have three copies of this card, in addition to any legal combination of the above cards.

The rule also applies to "A Legendary Ocean". Its card effect states its name is treated as "Umi" at all times, so you can only have three cards in total between these two. You cannot have three copies of "A Legendary Ocean" and three copies of "Umi" in your deck.

Example 2

"Proto-Cyber Dragon" only has its name treated as "Cyber Dragon" while it is face-up on the field. It is not illegal to have four or five cards on the field that all have the same card name, if the name change is only applicable while on the field and not in the Main Deck. So, you may theoretically have three copies of "Cyber Dragon", and three copies of "Proto-Cyber Dragon" in your Main Deck.

Example 3

• Cards that are similar in name and stats and depict the same monster. For example:
  • "Enraged Battle Ox" and "Battle Ox" have different card names.
  • "Obnoxious Celtic Guard" and "Celtic Guardian" have different card names.
  • "Swift Gaia the Fierce Knight" and "Gaia the Fierce Knight" have different card names.
  • "Black Luster Soldier - Envoy of the Beginning" and "Black Luster Soldier" have different card names.

In the above cases, a player may theoretically have up to three of each card above in their Deck. These cards are seen as Retrained from the original version.

Example 4

• Cards that are stored in the Extra Deck and have the same name as a card that exists in either the Main Deck or Side Deck:
  • "Neo-Spacian Marine Dolphin" is stored in the Extra Deck, but by its effect its name is treated the same as "Neo-Spacian Aqua Dolphin" and therefore you cannot have more than three of a combination of these cards, despite them being stored in separate Decks. "Neo-Spacian Twinkle Moss" follows the same rule with "Neo-Spacian Glow Moss".