I'd imagine 3 would be plenty for most sized TVs, but I'm not entirely sure how they would interact with a wall mounted TV. You'd need plenty of space between the back of the TV and the wall to make sure the Hue Play's lights can emit to the sides of the TV.
Otherwise, yeah controls will be found in the Philips Hue iOS app or Home app. You can automate them to turn on/off at certain times of day, or ask Siri as well yep. I'm not aware if you can sync them to the power state of your TV... I'm pretty sure not.
There is some science behind bias lighting color preference which you can find in a quick google search, but I tend to go with a very light blue. Some disagree, but I find it the most subtle and easy on the eyes in a dark room, while also still providing just the right light for navigating a dark room.