Modern metal production techniques often use solvent extraction either for separation or purification purposes. Processes were developed for specific mineral ores and deserve consequent modifications to be adapted to the recovery of metals from complex mixtures, arising eg from the hydrometallurgical processing of waste of electrical and electronic equipment (WEEE), as these mixtures contain a large panel of elements in highly fluctuating content. Three-liquid-phase (TLP) extraction systems allow the separation of metallic ions in three different phases in a single apparatus, and thus open the door to the treatment of complex mixtures with fewer operations. This paper reviews the work performed on TLP systems for metals separation, and highlights the potential advantages of such a technique in the design of new processes. Several systems enable the controlled formation of three immiscible phases, and have been successfully applied to the separation of various metals of interest. Advantages and drawbacks of each system are discussed, with a perspective for further developments, and the possibility for the rapid sorting of complex mixtures into separate groups of metals, easy to process.