To get in there, you have to get all 9 ranks on the rank screen. The 100%, Ultimate, and Speed in all three games - ROTH, OLB, and 3T.
In all three, the 100% rank is getting EVERYTHING.
In ROTH, the Ultimate is:
to complete without dying
only 10 hearts at the end
without talking to the Great Fairies
without getting bottles, and
without getting a shield
The Speed is beating the game in less than two hours.
You can do all three in one save if you do the Ultimate and Speed at the same time, then go back and find everything AFTER you beat it. You can pick up Pieces of Heart to heal, but if you get four, your max hearts will be 11 at end and you will not get ultimate. If you take the Master Sword, you will have at least 11 hearts and WILL NOT GET ULTIMATE. There is another way to beat Ganon, but it's a secret
In OLB, the Ultimate is:
to complete without dying
only 15 hearts at the end
without talking to the Great Fairies, not counting the ones that upgrade your sword to defeat Oni-Link.
without getting bottles,
without changing your tunic, and
without getting a shield
The Speed time is four hours in this game.
You can do all three in one save on this also with the same method: do the Ultimate and Speed at the same time, then go back and find everything AFTER you beat it.
In 3T, the Ultimate is:
to complete without dying,
only 12 hearts at the end, and
without getting bottles
The Speed time is seven hours in this game.
I hope this helps you.