The ONLY way to ensure equipment does not purge is by equipping your most important ones on at least one fighter. Everything else not equipped on one or more fighters is fair game to the purgatory. What is not entirely certain is whether the game purges based on oldest collected or least recently used. My initial theory was oldest collected, but someone with exactly 3000 pieces could test to see which is correct with the following method:
Go to Vault - Records - Stats and scroll about 60% down to see if you have exactly 3000 pieces of equipment. If so, go to customize a fighter, act like you are about to sell equipment, scroll to the bottom (right is faster than down BTW.) Once there, change the sort view by recently acquired, and take note of the last few pieces of equipment. Then change sort view by last used and also make note of the last few pieces of equipment, especially noting exactly how much the stats go up and down on those pieces.
Now go to the fighter who can use the bottom-most equipment based on last used view, equip it, and save. Go back to that fighter and unequip that piece of equipment and save again. This will effectively make your least recently used equipment become your most recently used equipment.
Now go into any game mode (preferably All-Star on Hard or an easier Master Orders ticket) and try to obtain just one piece of equipment. Did it purge your formerly least recently equipped item (which is now your most recent), your formerly second-least recently equipped item (which is now your least), or did it purge your least recently acquired piece of equipment? This may be the only way to know for sure how the game decides.
Unfortunately I cannot test this, as I now routinely go through my equipment, and sell my weakest equipment. Keeping my grand total in the 2000 to 2500 range ensures I have some breathing room when I play Crazy Orders so that I never hit that dreaded 3000 mark again, where who knows what will get purged.
