Update prereqs for Core 3.0 Preview 1

[Thraka]

MacOS, Linux, and Windows prereqs topics need to be updated for 3.0

• Windows: https://docs.microsoft.com/en-us/dotnet/core/windows-prerequisites
• macOS: https://docs.microsoft.com/en-us/dotnet/core/macos-prerequisites
• Linux: https://docs.microsoft.com/en-us/dotnet/core/linux-prerequisites