The most convenient way it seems to get .NET Core 3.0 installed on a Windows PC, is to download the free Microsoft Visual Studio 2019 Community edition. Installing Visual Studio 2019 will also install .NET Core 3.0 and after the installation is done, you can start playing around with it.
If you want to download the SDK or just the runtime separately, you can find them here.
To check if you already have .NET Core 3.0 installed, you can pull up a Command Prompt and run the dotnet --version command.
Iris Classon wrote a good lengthy post about the history of .NET web development and how it all lead to the development of the .NET Core that we have today. As someone who doesn't get to work as much on the web dev side of things, this was a very informative read for me. I think it is a good read for any .NET developer, so check out her post by following the link below.