TWO PAX STATED THAT THE FLT ENCOUNTERED TURBULENCE DURING THE APCH. THE ACFT LANDED HARD & THE LT MAIN LANDING GEAR COLLAPSED. THE ACFT THEN YAWED TO THE LT & THE NOSE & RT MAIN LANDING GEARS COLLAPSED DUE TO THE SIDELOAD. EXAM OF THE LT MAIN LANDING GEAR ASSY REVEALED THAT THE WALKING BEAM FAILED DUE TO OVERSTRESS WHICH ALLOWED THE GEAR TO COLLAPSE. EXAM OF THE ACFT REVEALED THAT THE RT ENG SEPARATED IN A DOWNWARD DIRECTION. THE LT ENG WAS PARTIALLY ATTACHED BUT DEFLECTED IN A DOWNWARD DIRECTION.