Who the hell are you?
The Third Gate is the last of three gates which must be cleared to win Halliday's Easter Egg Hunt. Once inside, the player must surpass a score of 728,329 points in the video game Tempest, and complete a Flicksync of the movie Monty Python and the Holy Grail. Completing both challenges places an avatar at The End, wherein they will find the Easter Egg.
Ready Player One
An OASIS avatar can only enter the Third Gate if they have acquired a copy of the Crystal Key and, inherently, a copy of the Jade Key and Copper Key. Additionally, there must be two other avatars present who have fulfilled the same conditions present at the gate. All three keys must be inserted into three keyholes, and one of the avatars must recite the phrase "3 is a magic number." in order to open the gate.
In the film, after Parzival retrieves the Easter egg in Adventure and gets the Crystal Key, the Third Gate rises out of the ground nearby. It is depicted as a giant block with three keyholes on the front for each of the collected keys. Once Parzival inserts and turns all of the keys, the front of the block where the keys were inserted shatters, revealing an opening to a massive room with a Golden Egg near the top and a desk with a contract on it.