Updating versions to preview3

NuGet.config

@@ -1,6 +1,7 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear />
     <add key="AspNetCore" value="https://dotnet.myget.org/F/aspnetcore-ci-dev/api/v3/index.json" />
     <add key="AspNetCoreTools" value="https://dotnet.myget.org/F/aspnetcore-tools/api/v3/index.json" />
     <add key="NuGet" value="https://api.nuget.org/v3/index.json" />

version.props

@@ -1,6 +1,6 @@
 <!-- This file may be overwritten by automation. Only values allowed here are VersionPrefix and VersionSuffix.  -->
 <Project>
   <PropertyGroup>
-    <VersionSuffix>preview2</VersionSuffix>
+    <VersionSuffix>preview3</VersionSuffix>
   </PropertyGroup>
 </Project>