[evulish]

1. Well, you could encrypt it using a method that needed a keyphrase (something like RSA or blowfish, I think..or something like PGP). Use a really long phrase of random characters, then store the characters in a file. Each year, get a new keyphrase and dump the 4th one. It wouldn't be the best way to do it, I'm sure (you could write it down then save it) but it would keep most people out.

2. Also, you could store files on one computer in a directory labelled for the year...then 3 years later, just shred that DIR.. you'd need to password protect that and make it read-only for the owner. You could store passwords in a file in a school office or some place that won't change. You could also have the user-account expire on the owner of those files after three years..

Both those ideas are moderatly easy to do.. the second being the easiest.

<edit>numbered ideas</edit>