Where is my CVV2 code (Credit Card Security Code) located on my card?
Visa and MasterCard credit cards have a three-digit security code number printed just above the signature panel in reverse italics on the back of your card. Usually it is preceded by your card number. American Express cards have a four-digit security code above the credit card number on the front of the card.

What is a CVV2 Security Code and why do you need them?
This code is used to establish that you are in posession of the card used for the payment. All MasterCard cards, both credit and debit, were required to contain CVC2 by January 1, 1997. Visa cards must have a CVV2 by January 1, 2001.

I found a number on the back of my card, but it has more than 3 digits.
We only need the last 3 digits of this number (the other are just a repetition of your credit card number).