Free-breathing continuous acquisitions, so called free-running, enable 3D whole-heart coverage for motion-resolved functional cardiac MRI. In prior work approaches based on 3D radial imaging were proposed for this task. However, free-running 3D radial image reconstruction is computationally demanding. In this work, we propose a novel 3D Cartesian free-running bSSFP sequence. Data is acquired continuously with a variable-density Cartesian trajectory with spiral profile order and retrospectively binned based on respiratory self-navigation and cardiac ECG signal synchronization. A multi-bin-PROST reconstruction is proposed to exploit spatial, cardiac and respiratory temporal redundancies to reconstruct high-resolution images.