Can I only return one card to the bottom of the deck from the trash with this card's [On Play] [When Digivolving] [When Attacking] effect?
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No, you can't. A "by" condition can't be met if only some of the required actions are performed. The conditions for this [On Play] [When Digivolving] [When Attacking] effect require you to return three cards to the bottom of the deck.
Source: BT26-016 Chronomon: Holy Mode