We present results of a combined X-ray/optical analysis of the dynamics of the massive cluster MACS J0358.8-2955 (z = 0.428) based on observations with the Chandra X-ray Observatory, the Hubble Space Telescope and the Keck-I telescope on Mauna Kea. MACS J0358.8-2955 is found to be one of the most X-ray luminous clusters known at z > 0.3, featuring LX, bol( < r500) = 4.24 × 1045 erg s-1, kT = 9.55+ 0.58- 0.37 keV, M3Dgas( < r500) = (9.18 ± 1.45) × 1013 M&sun; and M3Dtot( < r500) = (1.12 ± 0.18) × 1015 M&sun;. The system's high velocity dispersion of 1440+ 130- 110 km s- 1 (890 km s-1 when the correct relativistic equation is used), however, is inflated by infall along the line of sight, as the result of a complex merger of at least three subclusters. One collision proceeds close to head-on, while the second features a significant impact parameter. The temperature variations in the intracluster gas, two tentative cold fronts, the radial velocities measured for cluster galaxies and the small offsets between collisional and non-collisional cluster components all suggest that both merger events are observed close to core passage and along the axes that are greatly inclined with respect to the plane of the sky. A strong-lensing analysis of the system anchored upon three triple-image systems (two of which have spectroscopic redshifts) yields independent constraints on the mass distribution. For a gas fraction of 8.2 per cent, the resulting strong-lensing mass profile is in good agreement with our X-ray estimates, and the details of the mass distribution are fully consistent with our interpretation of the 3D merger history of this complex system. Underlining yet again the power of X-ray selection, our analysis also resolves earlier confusion about the contribution of the partly superimposed foreground cluster A 3192 (z = 0.168). Based on very faint X-ray emission detected by our Chandra observation and 16 concordant redshifts we identify A 3192 as two groups of galaxies, separated by 700 kpc in the plane of the sky. The X-ray luminosity and mass of the two components of A 3192 combined are <0.5 per cent and <8 per cent of that of MACS J0358.8-2955.