For Leopard V3, you need to get Leopard V2 and Rokushiki Fighting Style.
First, get an "Authority Chip" from Marine Raid on Teleport Island (First Sea).
Second, give the chip to S-Bear in Rogue Town to gain access to the Final Leopard Dungeon on New Future Island (Third Sea).
Next, go to the Final Leopard Dungeon to get the Feral Spirit (1% chance). The boss has 95 billion health and is using Final Leopard.
Finally, talk to the Final Leopard Trainer located at Destroyed Marine HQ with Rokushiki, Leopard V2, and Feral Spirit for Final Leopard.
WARNING: THIS WILL REPLACE YOUR CURRENT FRUIT