Ajax release 1.0 is an "extension" to the .NET framework 2.0
.NET 3.0 is an overlay of .NET 2.0 including the Windows Foundation stuff, mainly used in Windows Vista - which includes the WPF, WCF, WCS, and WWF
There wont be Ajax validator dlls in the .net 3.0 framework.
I'm assuming you are speaking of the System.Web.Extensions.Validators exception that gets thrown when you try to use ajax 1.0 released version.
Take the <tagMappings> section in your web.config and comment it out.