as others said, you have a year from your purchase date to decide whether or not to get it.
several things to remember:
1) it's not an insurance - it won't cover accidents
2) you get 3 years from the date of purchase of the computer, not the date of registration of applecare
3) for the first year, the original warranty that comes with the computer will cover any hardware defects. when you call after the 90 day phone support, you will be asked to give a credit card number in case the problem you are calling about turns out to not be hardware related. but if the problem is a genuine hardware problem, your credit card will not be charged.
4) for laptops, it makes a bit more sense, as one failed part could often mean total logic board replacement -> more expensive.