RPG Insanity

I'm going insane with the "Equip 3 types of Weapon" Panel Challenge.

I'm putting everything I get on everyone to try to get it to progress, but it's stuck at 1/3 no matter how many types of weapons I put on my characters. What specifically are they telling me to do?

You just need to be patient. When you advance the main story far enough, each party member will be able to equip a second and a third weapon. Simply play/wait until your party members can equip a third weapon and then equip one of them with three weapons to fulfill the achievement's condition.

If you check the equip screen, you should see that there are three weapon slots for each party member, and when you prematurely try to equip a second or third weapon, the game even tells you at which story quest (number) the slot will be unlocked.