Update pre release label to Preview 3.
Update pre release label to Preview 3.

StephenBonikowsky/wcf,imcarolwang/wcf,mconnew/wcf,mconnew/wcf,mconnew/wcf,KKhurin/wcf,StephenBonikowsky/wcf,zhenlan/wcf,imcarolwang/wcf,KKhurin/wcf,dotnet/wcf,dotnet/wcf,dotnet/wcf,imcarolwang/wcf,zhenlan/wcf