Decrypt 3DES with a key and a nonce?
Challenge-based symmetric-key authentication works a bit like this: 


Server picks a randomly chosen challenge and sends it to the client. 
Client "transforms" the challange with his key and send the result back to the server. 
Server does the same transformation with the same key, and verify if the two matches. 


The transformation can be an encryption algorithm, but usually well-designed system would use a MAC (message authentication code) such as HMAC. 

In your case, the nonce is most likely a challenge, although 3 character only provides 24-bit (&lt;21 bits if ASCII-only) insecurity.