As a test once I got X-Lite (free version of Bria) working with 3CX v15 by manually setting up the settings (Registrar address, extension username and password) in the client itself, so I would say yes it is possible with some 3rd party soft-phone models.
What I would say however is that 3CX's soft-phone in my view far surpasses all other versions out there on the market today. The features, functionality and time saving during setup that you will get makes it an obvious choice.