Ok. Here's a standard ASP.NET header when you are using a CodeFile. This is what Visual Studio adds to every ASPX page you create in it when you check "Place Code in Separate File"
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<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_Default" %>
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and in the codefile (default.aspx.vb)
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Partial Class _Default
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Inherits System.Web.UI.Page
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The partial class name must be the same as the file name, and must be the same as the "Inherits" attribute of the header in the .aspx page.
So in your case, I bet that your filename and class name aren't correct. The file should be test3.aspx.vb, and the class name in the file should be test3.
Make sure that you are doing this. If you are uncertain, make a new page in Visual Studio, making sure to check the "Place Code in separate file" check box, and paste your code in appropriately. If you don't have Visual Studio, Microsoft distributes the Express versions for free. The one you'd want is Visual Web Developer.