I've tried to setup my web application in in IIS 8 windows server 2012 and found that asp.net 3.5 or dotnet 3.5 not in IIS 8.
What i do is, go to add role and add in ASP.NET 3.5 in Web Server Role (IIS) -> Role Services, under Application Development. You can select others role if needed.
Click next to proceed until the installation complete.
In IIS 8, you can select .NET V2.0 Integrated or .NET V2.0 Classic in application pools for application that running is .net 3.5
To know more details, please visit http://www.iis.net/learn/get-started/whats-new-in-iis-8/iis-80-using-aspnet-35-and-aspnet-45