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 a new usurper, Leo III.. Theodosius ended his life in a monastery.
See G. Finlay, History of Greece (ed. 1877, Oxford), i. p. 396). (M. O. B. C.)