3-Packs
3-Packs is a format where you choose three modern-legal sets that saw a standard print run and make one pack of cards from each set. To make a pack, you choose 1 rare/mythic, 3 uncommons, and 11 commons; only one pack can contain a mythic, and each pack can only have one of any given card (duplicates can exist between packs, but not in the same pack). The rarity of a card is considered to be the rarity it is in the set it was printed (M14 Doom Blade is uncommon, but M12 Doom Blade is common).
Once you have your 45 cards, you use those cards and any number of basic lands to make a 60 card deck. Any cards not used in your main deck make up your sideboard.