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You can manage & track your card collection and decks once you create an account and login.

Can I get a card if I’m already playing with 3 of that card in my deck?

Yes. The 3-card and 25-star limits apply only to your starting deck and team. You can bring in a fourth card, even if it would put you over the 25-star limit.Allowing you to go over the 3-card limit was overruled for competitive play. See Tournament rules on sideboarding.
Source: Wizards of the Coast (2019-03-01)    【Link to Ruling】
Tags: Unleash Potential