The Nuffield Universal Three and Three Four tractors represent a significant era in British agricultural engineering, produced from the mid-1950s through the 1960s. These robust diesel-powered machines, typically featuring British Leyland or Perkins engines with power outputs ranging from 35-55 HP, were designed with a focus on reliability and versatility for a wide array of farming tasks. Their engineering quality is evident in their durable construction, manual gearboxes, standard PTO options, and often included differential locks for enhanced traction. The Nuffield tractors are recognized for their practical design philosophy and solid manufacturing, making them enduring workhorses in the agricultural sector. This comprehensive service and repair manual provides full coverage for the Nuffield Universal Three and Three Four tractors. It details engine overhaul, lubrication, cooling, fuel, exhaust, and starting/charging systems, alongside transmission components including the clutch, PTO, and hydraulics. The manual also covers chassis, steering, braking, electrical systems, and operator controls. With step-by-step instructions, troubleshooting guides, and hundreds of detailed illustrations, this resource is essential for owners, mechanics, and technicians to perform scheduled maintenance, diagnostics, and complex repairs, ensuring the continued operational efficiency and longevity of these classic tractors.