triumvir

Concise Oxford English Dictionary © 2008 Oxford University Press:
triumvir /trʌɪˈʌmvə, ˈtrʌɪəmvə/

▶noun (pl. triumvirs or triumviri /-rʌɪ/) (in ancient Rome) each of three public officers jointly responsible for overseeing any of the administrative departments.

– derivatives
triumviral adjective.

– origin L., orig. as triumviri (pl.), back-form. from trium virorum ‘of three men’, genitive of tres viri.