Where can I get the older version of silverlight 3?

A Silverlight 4 plugin will behave like a Silverlight 3 plugin when the XAP version points to its embedded Silverlight 3. This includes any bugs Silverlight 3 had where fixes for those bugs might break an existing Silverlight 3 application. This is called a "quirksmode" ( sounds familiar).

While it is best for bands and braces to test Silverlight 3 applications on a real Silverlight 3 plugin, you will likely find that the same issues are raised even if you are using the Silverlight 4 plugin.