Cryptanalysis of ciphertext using Java

I'm looking for some ideas on an assignment.

I have 7 ciphertext files, all of which are encrypted using the same symmetric key, which is 3 characters long and is alphabetic. No encryption algorithm is provided but the specs state that it is a home-made algorithm and is naive (whatever that means). My objective is to decrypt these files. I'm merely looking for ideas on the attacks which I can carry out on these files.

So far, I have done a frequency analysis, brute force attack to detect Ceasar Cipher, Krasinsky's method to detect Vigenere Cipher, Ciphertext XOR to detect a simple version of the stream cipher. I suspect that the files were encrypted using some mix of ciphers.

By the way, the decrypted plaintext is supposed to contain just a plain message, but the ciphertext reveals the use of over 97 different ASCII symbols!

Any general help, ideas or directions are greatly appreciated! Honestly, I'm not expected to decrypt these files, but then I might as well prove my professor wrong with your help. Thanks!

EDIT

I'm looking for attacks on block or stream ciphers. At least thats what I suspect...