You don't need to use a third party repository to install cmake-3. cmake-3 is available from Ubuntu Focal repositories (and Debian).
sudo apt install cmake sudo apt install build-essential libgtk2.0-dev libappindicator-dev libnotify-dev git clone https://github.com/nitroshare/nitroshare-desktop.git cd nitroshare* mkdir build && cd build cmake .. make ./out/bin/nitroshare-ui