There are many ways for you to get series 3 cards. First, you can get them from Collector's Cache and Collector's Reserve. Secondly, there's Seasonal series 3 card free claim. Also, you should be able to find many Series 3 cards and their variants from the shop bundles. Getting the variant allows you to use the card even if you don't own the base card yet! One more thing, you can find a Mystery Series 3 card from the Token Shop if you are missing any Series 3 card in your collection.