If I use the "reduce the play costs of all of your opponent's Digimon by 1" effect and all of my opponent's Digimon have their play costs reduced by 3, does an opponent's Digimon with a play cost of 2 now have a play cost of -1?

No, it doesn't. The minimum value is 0, the resulting play cost can't be negative.

If I use the "reduce the play costs of all of your opponent's Digimon by 1" effect, is the play cost also reduced for Digimon cards in areas other than the battle area (such as my opponent's hand or trash)?

No, it isn't. This card's effect only reduces the play costs of Digimon in your opponent's battle area.

Tankdramon rulings (BT16-060)

2 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

Effect: [On Play] [When Digivolving] Reveal the top 3 cards of your deck. For each [D-Brigade] or [DigiPolice] trait card among them, reduce the play costs of all of your opponent's Digimon by 1 until the end of the turn. Return the revealed cards to the top or the bottom of the deck. Then, delete 1 of your opponent's Digimon with a play cost of 4 or less.
Inherited effect: [All Turns] [Once Per Turn] When one of your other Digimon with the [D-Brigade] or [DigiPolice] trait is played, De-Digivolve 1 1 of your opponent's Digimon.

Official Q&A

If I use the "reduce the play costs of all of your opponent's Digimon by 1" effect and all of my opponent's Digimon have their play costs reduced by 3, does an opponent's Digimon with a play cost of 2 now have a play cost of -1?
No, it doesn't. The minimum value is 0, the resulting play cost can't be negative.
Ruled Mar 28, 2024
If I use the "reduce the play costs of all of your opponent's Digimon by 1" effect, is the play cost also reduced for Digimon cards in areas other than the battle area (such as my opponent's hand or trash)?
No, it isn't. This card's effect only reduces the play costs of Digimon in your opponent's battle area.
Ruled Mar 28, 2024

Got a line that uses Tankdramon? Map it in the builder and share it with a link.

Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.

Tankdramon rulings (BT16-060)

2 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

Effect: [On Play] [When Digivolving] Reveal the top 3 cards of your deck. For each [D-Brigade] or [DigiPolice] trait card among them, reduce the play costs of all of your opponent's Digimon by 1 until the end of the turn. Return the revealed cards to the top or the bottom of the deck. Then, delete 1 of your opponent's Digimon with a play cost of 4 or less.
Inherited effect: [All Turns] [Once Per Turn] When one of your other Digimon with the [D-Brigade] or [DigiPolice] trait is played, De-Digivolve 1 1 of your opponent's Digimon.

Official Q&A

If I use the "reduce the play costs of all of your opponent's Digimon by 1" effect and all of my opponent's Digimon have their play costs reduced by 3, does an opponent's Digimon with a play cost of 2 now have a play cost of -1?
No, it doesn't. The minimum value is 0, the resulting play cost can't be negative.
Ruled Mar 28, 2024
If I use the "reduce the play costs of all of your opponent's Digimon by 1" effect, is the play cost also reduced for Digimon cards in areas other than the battle area (such as my opponent's hand or trash)?
No, it isn't. This card's effect only reduces the play costs of Digimon in your opponent's battle area.
Ruled Mar 28, 2024

Got a line that uses Tankdramon? Map it in the builder and share it with a link.