There // They're // Their

To cast this split card, choose one of its thirds to cast. The characteristics of the thirds you didn’t cast are ignored while the spell is on the stack.

This split card has three names. If an effect instructs you to choose a card name, you may choose one of those names, but not all of them (though you might want to write it down and not just say it out loud).

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.

If an effect allows you to cast a spell with certain characteristics, consider only the characteristics of the spell you’re casting. For example, if an effect allows you to cast an instant or sorcery spell with mana value 2 or less from among cards in your graveyard, you could cast There or They’re this way, but There is right out.

This split card’s characteristics are a combination of its three thirds while it’s not on the stack. For example, There // They’re // Their has a mana value of 6 while it’s in your library. If an effect allows you to search your library for a card with mana value 4 or less, you can’t find There // They’re // Their.

Each split card is a single card. For example, if you discard a split card, you’ve discarded one card. If an effect counts the number of instant and sorcery cards in your graveyard, There // They’re // Their counts once, not three times.