You can't use them on the same game. You got 3 so you are entitled to buying 3 games from the list and using 1 coupon on each. Whether you can do it all on the same order with multiple games or not, I am not sure myself.
Actually it seems Xillia did very well for its first month. It did just over 100k by early September which is not marginally better than Graces or much worse than Vesperia.
Does it really end Sunday night or will it be when the store updates on the 2nd? I checked the promo again, and it says your purchases through April 1st.
http://blog.us.playstation.com/2013/02/26/psn-march-savings-get-10-for-every-50-spent/
at the very bottom.
Redeem code in the message. In fact when I opened the message it took me into the PSN store and entered the code automatically. This is my first time getting a code from them so I have no idea if that is standard.
Yes, with a credit card they just put a hold to make sure you actually have the money/real account. They will actually charge you when it ships, or very close to it.