Things 3 can sync all your information across your various Apple devices, but you still need to buy the app for each device. Things 3 works the same way across all platforms, but Cultured Code put a lot of thought into designing each interface specifically for each device.
So, Things 3 costs $10 on the iPhone, $20 on the iPad, and $50 on the Mac.
Mac users also get a free 15-day free trial, so you can see if you actually like Things 3 before dropping a 50-bomb.
My advice: Consider which devices you use the most, and go from there. Personally, I use my iPhone and MacBook more often than my iPad, so if you're like me, I would highly recommend getting Things 3 for those two devices at least — to keep yourself organized while at work or on the go. But if you love your iPad, Things 3 works great there, too.