Platinum alone sells for like $39 alone at GameStop, so that's good for all three. But I would be wary about buying online, especially when those games are priced so low. Could possibly be fakes.
If things turn out okay though, I highly recommend playing Platinum over the other two. For $30 for the three, you can't get much better than that and you won't be losing out on money if you ignore Diamond and Pearl in favor of Platinum. But DP just run...really slowly, especially when surfing and battling (even with the animations off). Platinum fixed that, as well as a bunch of other issues people had with Diamond and Pearl, and it's a really solid game.