Which consoles + games will I need to play the entire Kingdom Hearts series when KH3 is released?

The I.5, II.5, and II.8 collections cover most of your bases, so that should save you some money. I.5 and II.5 are available on PS3 and include the following full games:

I.5:

• Kingdom Hearts 1 Final Mix
• Kingdom Hearts: RE: Chain of Memories

II.5:

• Kingdom Hearts 2 Final Mix
• Kingdom Hearts: Birth By Sleep Final Mix

The II.8 collection will include only one remaster, and two new titles, so this is a must for completion purposes. This will be a PS4 exclusive title.

II.8:

• Kingdom Hearts: Dream Drop Distance Remastered
• Kingdom Hearts X: Back Cover
• Kingdom Hearts 0.2: Birth By Sleep - A Fragmentary Passage

Games that are not included in these collections are as follows:

Nintendo DS:

• Kingdom Hearts: 358/2 Days
• Kingdom Hearts: RE:Coded

Note: By purchasing the collections, you are saving yourself from buying a Nintendo 3DS if you don't already have one.

As you probably know, Kingdom Hearts III will be on Xbox One and PS4.

To play all of the games with minimal consoles, you would need a PS3, PS4, and Nintendo DS.

At this specific point in time, you'll need two systems:

1. PS3
2. 3DS

The games are pretty straightforward as well:

1. Kingdom Hearts 1.5 ReMix
2. Kingdom Hearts 2.5 ReMix
3. Kingdom Hearts Dream Drop Distance

This will give you the whole story as we currently know it. Come December, though, this list changes slightly. Swap the 3DS for a PS4, because that's when Kingdom Hearts 2.8 ReMix comes out. You also swap Dream Drop Distance for the 2.8 ReMix. Additionally, 2.8 supposed to have some extra content you can't find anywhere else, so that would be providing the most lore throughout. Kingdom Hearts 3 is supposed to come out some time next year (although I kinda doubt it), and that's going to be for PS4 only, I believe. So getting 2.8 makes sense for preparation for KH3.

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If you absolutely, positively must play every single game (since the ReMix versions turn a couple of them into just story cutscenes), the list changes a bit.

Systems:

1. PS3
2. DS
3. 3DS

Games:

1. Kingdom Hearts 1.5 ReMix
2. Kingdom Hearts 358/2 Days (DS)
3. Kingdom Hearts 2.5 ReMix
4. Kingdom Hearts Re:Coded (DS)
5. Kingdom Hearts Dream Drop Distance

That will allow you to play all the games, with the additional caveat above about 2.8 replacing Dream Drop Distance in December.