You’ve got plenty of time to get caught up on Kingdom Hearts before Kingdom Hearts III comes out next January, and preorders just went live for Kingdom Hearts: The Story So Far, which comes out at the end of October, and includes HD remasters of every previous title. It’s not a deal, but it’s possible the physical copy could sell out.
As a separate reminder, Prime members can also preorder Kingdom Hearts III to receive a $10 Amazon credit (30-35 days after the game comes out). That code is available whether you order a physical or digital copy.