Gas turbine engines including multi-axis accessory gearboxes of mechanical drive systems are provided. The gearbox comprises a housing, a drive shaft, bevel pinion and drive shaft bevel gears, and a side bevel gear set operable to directly at least one side accessory device. Housing is disposed about a towershaft operatively coupled to main engine shaft and operable to rotate about a first axis. Drive shaft is skewed to main shaft and operable to rotate about a second axis that intersects the first axis at a first angle. Bevel pinion gear is mounted on the towershaft. Drive shaft bevel gear is mounted on the drive shaft. Side bevel gear set comprises an input gear meshing with one or more side bevel gears each having a side bevel gear axis at a second angle to the first axis and independently positionable relative to input gear. Second angle is independent of other angles.