many website share just one database file ?

[micky]

there are three wesite called A,B,C

these are just share 1 database file called database.mdf in app_data

it's that i wanna make design. i don't want to use sql server machine. just
use database file

it is not working...
<add key="Conn" value="Data Source=.\SQLExpress;Integrated
Security=True;User Instance=True;AttachDBFilename=A\App_Data\Database .mdf"
/>
and there are some error...
An attempt to attach an auto-named database for file A\App_Data\Database.mdf
failed. A database with the same name exists, or specified file cannot be
opened, or it is located on UNC share.

please help....~~