[1.12.2] Custom Inventory slot

[Kopselek2]

I want to add new 3 slots to minecraft vanilla inventory, i dont wanna make new inventory and replace it, because im using customnpc on server and i will override it that way, i want to add slots for custom rings and custom amulet. 

[DragonITA]

1. 1.12.2 is no longer supported due to his age.