Kingdom Hearts 3 will not just take you into the last instalment of the dark secret saga but also other games in the franchise. This is through Memory archive which will have videos showing the story of KH so far. (Considering the wait and with all the games that has come out many people might need it) If you need to catch up you can check out the videos below.
Article:https://gematsu.com/2019/01/kingdom-hearts-iii-memory-archive-english-summary-movies
As always thank you for reading.