Three Myanmar soldiers were killed in a clash with a group of armed people on Mekong River during an international joint patrol, the Bangkok Post reported on Monday, citing security sources.A joint patrol force of Myanmar and Lao soldiers clashed on Sunday with the gangsters who reportedly belong to a drug trafficking group about 20km north of the Golden Triangle, the sources said. China, Laos, Thailand and Myanmar began their coordinated patrol on the violence-troubled Mekong River last Saturday following the killing of 13 Chinese sailors on the Thai section of the river in October.Nine Thai armymen have been charged with murder and tampering with evidence in connection with the killing of the Chinese sailors.