* Assad expected to seek third presidential term
* Foreign critics have called vote a "parody of democracy"
* Infighting has weakened Assad opponents (Adds U.N., U.S. comments, U.S. indications of toxic chemical use, paragraphs 9-11, 17-19)
By Alexander Dziadosz
BEIRUT, April 21 (Reuters) - Syria announced on Monday a presidential election for June 3, preparing the ground for Bashar al-...