This is an example of default.asp that will redirect the requests for different domains 
to appropriate directories.

<%
If InStr( UCase(Request.ServerVariables("SERVER_NAME")),  UCase("domain1.com") ) > 0 Then
        Response.Redirect("/directory1")
ElseIf InStr( UCase(Request.ServerVariables("SERVER_NAME")), UCase("domain2.com") ) > 0 Then
        Response.Redirect("/directory2")
ElseIf InStr( UCase(Request.ServerVariables("SERVER_NAME")), UCase("domain3.com") ) > 0 Then
        Response.Redirect("/directory3")
Else
        Response.Write("You came to an unknown domain.")
End If
%>

Notes:
1. Replace "domainX.com" with the actual domain names.
2. Replace "/directoryX" with the actual names of the sub-directories of your
   /Htdocs directory.
3. There is no need to replace "SERVER_NAME" with the name of the primary
   domain of the website. SERVER_NAME is a meta variable.
4. You can use specific file names instead of directory name in "/directoryX".
5. Remove all comments in this file before making it actual default.asp.