With Silverlight 5 knocking on the door, we don't have any plans to support and develop controls for Silverlight 3. My suggestion is to use the Silverlight 4 version of our controls, which is the one we are currently developing and supporting.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!