I doubt there is any reason to purchase more than one game. I'm sure it was easy enough for Chunsoft to tweak them into three different versions and hope people will get confused and think they need to buy all three.
Well, it's probably the same engine and graphics and stuff. But I bet that each game has different content to it. Also, thinking about it, it totals up to 3600 Wii Points for all 3, which is about the price of a new DS game. It's just broken up into 3 segments.