Rider doesn't see NetCore 3.0

It is pretty common problem. I have solved it by specifying path to .Net Sdk manually. This is the screen of settings:

1. Specify path to .NET binary file. It is usually has the following path /home/<your_user>/dotnet/dotnet

2. Specify path to MSBuild file. For example my path is the next : /home/aleksej/dotnet/sdk/3.0.100-preview6-012264/MSBuild.dll

3. Save

4. Profit!

P.S. My operation system is Linux Mint 19.1. And version of .Net Core is not important in this case.

Update

I forgot about one important moment. If you need to change .NET version for your solution, you have to place special global.json file in the root of your solution. If you are changing .NET version for a project - put global.json on the project root path.

Example of global.json for changing .NET version:

{
  "sdk": {
    "version": "2.2.300"
  }
}

If you want to find the location of your sdk then write the following command: dotnet --list-sdks. It has to show you 3.1.403 [/usr/share/dotnet/sdk] or similar to it. If it's not then consider it installed using the following guide Install .NET Core on Linux . Then select the dotnet file in .NET CORE CLI executable path, with it's respective MSBuild.dll .
PD: Remember to save the changes globally!!!.