Okay, probably already answered in full, but i'll go into it too.
In terms of weird naming schemes, yeah Kh is weird. Its not impossible though so here it is:
Of the kh games we have:
1 [ps2]
chain of memories [gba](and the ps2 remake re:chain of memories)
2 [ps2]
358/2 days [ds](side story set around the time of chain of memories)
birth by sleep [psp] (prequel)
coded[old ass mobile] (remade on ds as re:coded) (also a side story, but super not important)
3D (dream drop distance)[3d](prequel to kh3. basically ruined the plot)
χ / unchained χ [pc browser/ mobile game] (takes place wayyy before everything.)
so on ps3 we never had a proper release of a new kh game. instead, we got a couple hd collections.
1.5 (includes 1, re:chain of memories, and cutscenes from 358/2 days)
and
2.5 (includes 2, birth by sleep, and cutscenes from re:coded)
now on ps4, prior to kh3's release, we're getting 2 more hd rereleases:
2.8 (3D and 2 new pieces of content. 0.2 birth by sleep(takes place after birth by sleep), and a movie called χ back cover detailing all the events of χ
and 1.5 + 2.5 (literally everything in 1.5 and 2.5 but in 60fps)
so... basically to catch up in the series, just buy 1.5 + 2.5 and 2.8 and you are golden. only need 1 console too to experience the whole series now :3