have you a link to where you sign up for this? do u only need to cancel by the end of the 3rd month not to be charged again at the full price?
When you go to subscriptions it gives the option for promo/prepaid codes. Stick in 'xboxskysports20'. You pay the full €30 for 3 months up front.
Once your signed up though you can cancel straight away so you wont be charged more after the promotion period.