Hello fillot,
The source code for OpenVPN Connect v3 is not open source. However the library used for the VPN tunnels is. That is the openvpn3 project found on github here:
https://github.com/OpenVPN/openvpn3
So basically the graphical shell around it is proprietary software that is designed to integrate well into our commercial solutions, but the software making the VPN tunnel is open source.
If you're looking for an open source client you can check into OpenVPN GUI or Tunnelblick or OpenVPN for Android for example.
Kind regards,
Johan