Sheppard was also ordered to pay a total of 12 million riel (£2,000) in compensation to the three victims and be deported from Cambodia after he finishes his sentence, Action Pour Les Enfants said.
Police believe the ex-air traffic controller used cash and gifts to tempt the young boys into going with him to his flat.
Once there, it is thought the father-of-two paid the children around £7 to have sex with him, and gave four children bicycles.
Cambodia is a magnet for so-called ‘sex tourists’ from across Europe, due to the country’s high level of poverty and poor child protection laws.
Notorious paedophile Gary Glitter, 71, fled to Cambodia after being caught in the UK in 1997 with 4,000 graphic images of children on his computer.
David Fletcher, from Norwich, was also convicted of twice raping a 17-year-old girl in Cambodia.
