The error code you're seeing is Authorize.net's error code, not x-cart's.
The first number is the response code: 3 = There has been an error processing this transaction.
The second number is the response reason code: 103 = This transaction cannot be accepted.
In Authorize.net's documentation there is an additonal note for error code 103: A valid fingerprint, transaction key, or password is required for this transaction.
Not sure exactly what that means. Sorry.
To get to the documentation I quoted...
go to
https://secure.authorize.net/
Log into your account
Click on "Help" (on the left)
Click on "Documentation and Reference Guides"
Click on "Advanced Integration Method (AIM)"
Good luck.