Similar to human health, the importance of hydraulics sometimes becomes apparent only in her absence. Images and motives of agricultural workers from the past times and centuries can look attractive only on paintings of arts. In reality, it was a very hard and sometimes quite dangerous job. The introduction of hydraulics in construction of modern farming machines, agriculture is less demanding for all farm workers, but also more efficient, which is of particular importance for providing enough food for the growing world during the 20-th century. Achieving high efficiency, reliability and long service life of these systems requires sophisticated diagnostics and failure repairment, regular maintenance and careful use of each hydraulic system, and this is only possible with a good knowledge of the functional principles of all elements, their connections and the whole system. However, modern hydraulic systems of agricultural machines are often very complicated, and the construction of their components is too complex for detailed graphic representation within the whole corresponding hydraulic installation. Therefore, instead of detailed representation of the components, schematic diagrams are used. This manuscript represents a logical continuation of the first and second parts of the paper, devoted to the general symbols, labels of measuring instruments and indicators (part one), as well as pumps and actuators (part two). The third) part of the paper presents the hydraulic symbols of the system control elements - hydraulic valves. The graphical symbols of these components are defined by ISO standards.