Dude. BBS and 358/2 are spin-offs, yes. But they are most definatly a part of the real series. Just as much as "Kingdom Hearts: Chain of Memories" was, but then again, you didn't play that did you? Kingdom Hearts 1 lead straight into KH: CoM, and a lot of people who hadn't played CoM didn't know shit about what was happening in KH 2. So if you want to at least understand half of what is happening in Kh3, you should try to play them, or show interest in them.
KH: Birth By Sleep ~ KH:1 ~ KH: CoM ~ KH:2 ~ KH: Coded ~ KH:3
....................................l 358/2 Days l
358/2 Days during Sora's adventure in Castle Oblivion, and is the story of Roxas' time in the Organization.