Pretty self-explanatory, I would like to know how can I get the zombies map The Giant on Call of Duty Black ops 3 on Pc as well as if it's released yet.
It appears that if you buy the Season pass (available on steam), it comes with "The Giant" zombie map. From the link:
About This Content The Call of Duty®: Black Ops III Season Pass delivers 4 epic DLC map packs in 2016 with all-new multiplayer content, including re-imagined fan-favorite maps from Call of Duty®: World at War and the Call of Duty®: Black Ops series, plus thrilling Treyarch Zombies content, all for one great price.* Fans that order the Season Pass will be among the first to experience the return of Zombies with “The Giant” Map in all of its undead glory on Day One. Set in a World War II-era research facility featuring the weapon-upgrading Pack-a-Punch Machine, re-live the chaos of Treyarch's classic "Der Riese" Zombies map, picking up the Zombies story with Dempsey, Nikolai, Richtofen, and Takeo where Origins left off.
Purchasing the Deluxe edition also gives it to you (since it comes with the season pass).