It's an exercise: the description contains all the information you need (including the algorithm) to decode the file.
Quote:
I've tried to guess it but couldn't.
You aren't supposed to guess: you are supposed to use a brute force attack approach to decrypt, which isn't hard or too time consuming given that the key you are looking for is three lower case characters - i.e. has only 26 * 26 * 26 possible combinations and the decryption result is text using only English words.
Give it a try, and see how far you can get...