You no longer need to. (For new people: in WSSv2 you did)
WSS 3.0 looks for existing files on the file system and virtual directories in the metabase when process starts and automatically decides not to handle them.
Note that in some circumstances this does not work because of ASP.NET rather than WSS issues.
Here is some info on how to solve this (which may later be in the WSSv3 Admin Guide)
This requires the following manual intervention by the IIS administrator:
1. Make an IIS Virtual Directory beneath the root.
2. In the IIS Manager, right click on the new Virtual Directory and go to Properties.
3. Click on the Virtual Directory tab.
4. In the Application settings section at the bottom, click Create to make this virtual directory an application.
5. Click the Configuration... button.
6. Remove the wildcard script map at the bottom of the window.
7. Click OK.
8. Click OK again.
9. Create a new web.config at the root of the new Virtual Direcory file system path and add the following nodes:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>