Yes. As a matter of fact, the Silverlight 3 API is considered feature complete in the Moonlight 4 Preview 1 version:
Release Notes Moonlight 4 Preview
Moonlight 4 Preview 1 is the first preview of Moonlight that includes support for the Siverlight 3 APIs and partial support for Silverlight 4 APIs. We consider the Silverlight 3 API support to be feature complete while the Silverlight 4 APIs are both partial and might be missing features.
[From the release notes.]
You can download it here. However, it's possible that some sites do stupid system checking based on your user agent - so it might be possible that you technically could view the content, but the site prevents it from being displayed. Just post the site in question.