How do I acheive this

Question

Hi,
In my current requirement, I am supposed to develop a software for a company. I have opted for .net and sql server.
The company has just three branches inside the city, I have opted for sql server express edition as its free.
Its going to be a desktop application, and not an web application.
Now I want that all three branches should interact the same database. I heard that using assembly.codebase we can achieve this.
Because as its a free edition the database will be in the main branch and the rest of the two branches have to deal with this database.
Kindly let me know how to do it or pls provide me with links for the same.

Regards
cmrhema