THEODOSIUS III  ., emperor of the East (716–717), was a financial officer whom a Byzantine army rebelling against Anastasius III. unexpectedly proclaimed monarch in his stead . He captured Constantinople after a six months' siege and deposed Anastasius, but in the following year was himself forced to resign by anew usurper, Leo III . (q.v.) . Theodosius ended his life in a monastery . See G . Finlay,' History of Greece (ed . 1877, Oxford), i. p . 396) . (M . O . B .