Don't forget, these three games are developed by three seperate teams.

FF XIII is the main FF team, who last worked on FF X-2 and since then have slightly split up to randomly help out in the Kingdom Hearts stuff and the FF VII Compilation stuff.

FF V XIII is the Kingdom Hearts team.

FF A XIII is the team from FF VII: Before Crisis and FF VII: Crisis Core, and they will make their fourth game Kingdom Hearts: Coded next.