TVA is the best way to go. But if for whatever reason you decide to play any other order, make sure you play Aqua last. Not because its bad, just because Aquas ending spoils Ventus's ending. Overall though, it is better to play TVA because the Final Episode and the secret movie Blank Points pretty much revolve around Aqua, so you want to have Aqua's ending fresh in your mind when you play Final Episode. As for simultaneous play, its possible, but it would be a drag to have to turn off the game (or hit the home button) and restart every time beat a world. Just enjoy the stories one at a time the way they were meant to be.
Also gameplay wise, Terra is slower than the other two but much stronger. Ventus's fighting style is light and speedy. Aqua is much more magical. Her attack strength is fine but her defense is lacking so she makes up for it with an all-around barrier instead of just a simple block. The point I'm trying to make is that if you play Ventus first, you will be too used to his speedy attacks and gliding so then when you play Terra, you will be disappointed because he is slow and has no glide ability. But if you play Terra first, then Ventus, you will be delighted to see that Ventus is fast and can glide. Then after you finish Terra and Ventus and feel like Aqua's probably not going to be special, you will be surprised to see that she is actually a refreshing character to play as. With her upgraded magic and all-around block, but low defense, you will see that the old hack-and-slash strategy wont cut it. At the beginning when she is still weak, if you're not careful, you'll get killed pretty easily (I only played Proud mode so it might be different on lower difficulty levels). So you have to use her unique abilities wisely.
So I say just play them one at a time TVA. Nomura himself said this was the best way to play it. I also recommend playing Proud mode on your first playthrough. Its really not that hard, and its much easier to unlock Blank Points that way. Unlike all the previous games, this secret movie is not just some vague concept trailer for the next game. It is the true ending of the game and is very important with huge impact in the story while also hinting at what's next to come. A must see for any KH fan. Sorry for making an essay out of this but it was necessary to get all my points across. I hope this helps you in your decision.
. Ventus play style seems wildly different, I can't catch the hang of it.
