Hi Brian Hiatt,
I think you may have already discovered this based on your download history today, but we have two separate installers available for Silverlight 3 (VS2008) and Silverlight 4 (VS2010). In order to get the Silverlight 3 compatible build, please use the link in the red box in the following image:
Otherwise, when downloaded you should notice that the file says "RadControls_for_Silverlight_3..." or "RadControls_for_Silverlight_4...". In your case, you would want to ensure you are installing the RadControls for SL3 version.
Please let us know if you have any other questions.
the Telerik team
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