Install .Net Core 3.1 SDK on Ubuntu 20.04 LTS (Focal Fossa)
Install Dotnet Core 3.1 on Ubuntu 20.04 LTS

On April 23rd 2020: Canonical announced the general availability of Ubuntu 20.04 LTS (Focal Fossa), which boasts top security and great performance.
For .NET Core developers here is how you can install the .Net Core SDK 3.1.
Install .Net Core
First download .NET Core SDK 3.1, you can download it here
Go to the directory you just downloaded the SDK 3.1 file. In my case, it is the Downloads folder.
$ cd ~/Downloads
Now, extract the SDK file and copy it to a directory named dotnet in your home folder using the following command.
$ mkdir -p $HOME/dotnet && tar zxf dotnet-sdk-3.1.201-linux-x64.tar.gz -C $HOME/dotnet
Configure .Net CLI
Add .Net core to your path to be able to run dotnet commands from your terminal.
$ export DOTNET_ROOT=$HOME/dotnet
$ export PATH=$PATH:$HOME/dotnet
The two commands above only make dotnet CLI commands available to your current shell session.
To permanently add the commands to your shell. You can edit your bash profile, which resides in : ~/.bashrc .
Open the bashrc file and add the line for exporting your dotnet to $PATH you used above, i.e export PATH=$PATH:$HOME/dotnet
, to the bottom of your bashrc file. Then save the file.
$ sudo gedit ~/.bashrc
After you add the line, the bottom of the bashrc file should look like the one below, i.e line 118.

Finally log out then log in again to apply the changes.
Conclusion
You can test if the dotnet CLI was configured successfully by running the command below.
$ dotnet