The only way to access this is by cheating. Here's what to do.
HOW TO TOGGLE CHEATS.
If you're using ePSXe: read the description of this video: • ACE COMBAT 3 electrosphere | Secret Menu
If you're using Duckstation:
Run the game;
Go to Settings/Cheats/Cheat Manager;
Click on "Add Cheat";
In the description type whatever you want; in the blank space type this code:
80112FB4 0002
8016E928 0500
Save the cheat.
1) Get a JAPANESE unpatched version of the game. No Project Nemo, just the normal Japanese version of the game, or else it won't work.
2) Begin a new game, use the default name "Nemo".
4) Set the difficulty to "Hard", don't change any setting, save the account on any one of the 3 slots.
5) Skip or watch the cutscenes and dialogues, whatever.
6) When you're in the pre-mission menu, go to "Options".
7) ACTIVATE the cheat, 3 hidden menus should appear.
8) Highlight any of the options, then DEACTIVATE the cheat.
Enjoy!
P.S.: Don't navigate the model display too fast, or else the game crashes.
Scrolling past Ouroboros's Delphinus 3, crashes the model display.
32