I've recently beat the single player campaign, and I got 2-slot gear from beating the final boss. I've also gotten some 2-slot gear from beating the amiibo levels. However, when looking at the equipment, I definitely see 3 slots, so I'm led to believe that it's possible to get 3-slot gear.
By 3-slot, I mean gear that has 3 additional slots that can gain abilities, bringing the total to 4.
How do I get 3-slot gear?