To create or open applications targeting .NET Core 3.0, Visual Studio 2019 or newer is required. Click the following link to download the latest version of Visual Studio 2019. If you are after an offline installation follow these instructions on how to install offline.
When creating a new project Visual Studio may show you a yellow bar with the message "ASP.NET Core 3.0 or newer projects are not supported by this version of Visual Studio"
Workaround: You need to hit 'Cancel' in dialog and then restart VS for the yellow-bar message to go away and ASP.NET Core 3.0 to show up in the dropdown. Simply cancel and re-open dialog without restarting VS does not fix the issue.