Thank you for writing.
Currently, RadRichTextBox can be used only with .NET Framework 4.0 and for the time being there is nothing that we can do about this. There is a request in PITS, where you can add your vote for this, and once it gathers considerable demand, we will consider the implementation of the feature: http://www.telerik.com/support/pits.aspx#/public/winforms/10500
I hope that you find this information useful. Let us know if you have any other questions or suggestions.
the Telerik team