The lowest caste of the mentioned groups are the Shudras.
The Shudras are considered to be the fourth Varna (the Sanskirt word which with multiple meanings but can be translated to character, or quality for example). According to the Rigveda (a sacred text in Hinduism) the job of Shudras was to serve the other three Varnas (Vaisyas - skilled traders, merchants, and minor officials, Kshatriyas - warriors and rulers, and Brahmans - priests).