I'm sure that they are all going to be released on both PS4 and PS5. Now that KH3 is released, my guess is that all of that staff, and probably some additional staff, are fully working on this remake. Episode 1 took forever to come out, partially because they started over not too long ago, but EP 2 and 3 shouldn't take near as long. SE knows this can be a cash cow.
Personally, I think it would be in their best interest to release these for $40/$50 a piece, instead of $60, since it is one game split into 3 pieces. And I imagine there will be an exclusive compilation of all 3 on the PS5 when the 3rd one comes out. I also think it's an almost 100% chance they will have cross saves for the different episodes and consoles.