Automatic Values Based On Other Fields

I've put together a database for my company to track our archived files. When the files are archived, there are three important values pertaining to their location: a list number, a box number, and a barcode number.
Each box has a barcode on it, so what I'd like to do is make it so that when any record has both a list and box number entered (since it is dependent on the list number to know which, for example, "Box 1" I'm entering information for), it would automatically insert the appropriate barcode number in the field. It would obviously need a seed for each barcode number to be entered before it would know which one to use, but that could either be based on the first unique entry, or entered another way. If anyone can help me do this, I'd really appreciate it. Also, if my description was insufficient or confusing, just let me know and I'll try to clear it up. Thanks!

Example:
If I entered:
List # Box # Barcode #
77 4
It might not know what the barcode is the first time. But once I had entered
List # Box # Barcode #
77 4 366582
It would know what I was entering. Keeping in mind that I'm not showing the rest of the record, but just this end part, my next entry would look like:
List # Box # Barcode #
77 4 366582
77 4 366582
...the red representing that it would automatically fill that in, because it knows that if I'm talking about Box 4 in List 77, that must be the barcode.