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?
You can talk to Spyke and give him a Super Sea Snail (which you can buy from him for 30k coins or receive from Splatfest participation). He then presents you the option to upgrade your gear or re-roll the special stats on it. You must be level 20 to buy the Sea Snail, though.
This gear has also been showing up in the shop at level 13. The items cost 8k-12k apiece.
Alternatively, talking to Spyke after reaching level 5 will unlock the ability to order gear that other players in your plaza are wearing. Gear ordered in this way will be available from Spyke one day later at approximately three times its shop price.
You will also receive unique 3-slot gear from Judd at levels 25, 30, 35, 45, and 50. Splatfest tees have three slots too, but disappear when their respective Splatfests end.
