Accurate centre-of-mass (CoM) alignment is essential for ensuring the validity of air-bearing spacecraft testing facilities used for ground-based verification of Attitude Determination and Control Subsystems (ADCS). Even small CoM offsets relative to the platform’s mechanical rotation axis introduce parasitic torques that distort detumbling and pointing experiments. To address this, we present the design, implementation, and validation of an autonomous mass-balancing system for planar air-bearing spacecraft platforms. The proposed system employs four motorised masses to regulate the CoM in three degrees of freedom. PID controllers independently adjust the X–Y planar mass positions, driving the CoM toward the platform’s rotational centre, while a Batch Least Squares (BLS) estimator identifies Z-axis imbalance and iteratively refines the mass configuration to compensate for out-of-plane disturbance torques. This hybrid PID–BLS approach enables autonomous balancing without iterative manual procedures. Experimental results show that the system reduces planar CoM error to within a millimetre and accurately estimates Z-axis imbalance under sensor noise and small disturbances. When activated, the mass-balancing mechanism significantly improves detumbling repeatability and stabilisation accuracy during ADCS experiments. By providing a low-cost, autonomous means of correcting multi-axis mass imbalance, this work enhances the fidelity of ground-based ADCS verification and supports safer, more reliable development of small-satellite technologies in increasingly congested orbital environments.