We present a joint cosmological analysis combining data from the Planck satellite, the Atacama Cosmology Telescope, and the South Pole Telescope, constructing a unified likelihood that reproduces the measured temperature and polarisation power spectra by jointly modelling the cosmic microwave background (CMB) signal, galactic and extragalactic foregrounds, and instrumental systematics across all datasets. This approach reduces reliance on external priors and improves the robustness of parameter estimation. Within this joint analysis, 
CDM parameters exhibit remarkable stability with respect to variations in foreground modelling. Extended cosmological parameters are more sensitive to these assumptions, with uncertainties increasing by up to 35%. Despite this, the combined constraints show no significant deviation from CDM expectations, and several previously reported tensions — such as the preference for non-zero curvature or the excess of lensing amplitude A_L — are significantly reduced or resolved. In contrast, the determination of foreground parameters more severely depends on the assumptions made about the underlying models. Overall, this work demonstrates the feasibility and reliability of a fully joint analysis of current CMB experiments, and emphasizes the importance of consistent and accurate foreground modelling for the scientific goals of next-generation, high-sensitivity CMB surveys.