You must play:

1-Kingdom hearts 1 final mix

2-kingdom hearts re chain of memories

3-kingdom hearts 2 final mix

4-kingdom hearts 358/2 days

5-kingdom hearts birth by sleep final mix

6-kingdom hearts re coded

7-Kingdom Hearts HD Dream Drop Distance

8-Kingdom Hearts 0.2 Birth by Sleep A Fragmentary Passage

9-Kingdom Hearts χ Back Cover

10-Kingdom Hearts Union χ or watch Everglow

11-KINGDOM HEARTS III Will be released worldwide in 29 January 2019

Note: You can buy an Kingdom hearts all-in-one package in PlayStation Store, PlayStation 4

just 70 days :).