We need to purchase a latest version of Infragistics NetAdvantage .NET, which supports .NET 2.0, 3.0, 3.5.
Richard,
The best place to get this information is the Supported Environment Overview. Please also note that in some cases this is also dependent on the platform. For example the last version of NetAdvantage for Windows Forms that supports .NET 3.5 is 2012 Volume 1 while the last version of NetAdvantage for ASP.NET that supports .NET 3.5 is 2012 Volume 2.
Let me know if you have any questions.