Roughly 10 gunmen arrived late Friday night at a house in the border town of Mandera where the workers were staying, shot the watchman in the head and kidnapped the three, the official said, speaking anonymously because he was not authorized to talk to the media.
He would not say who the victims worked for or what their nationalities are.
The watchman was hospitalized with life-threatening injuries, the official said.
The workers' kidnapping follows the seizure earlier this week of two French security advisers in the Somali capital.
The French advisers were on a mission to train Somali government forces, which are fighting Islamist militiamen. They were abducted Tuesday from a hotel in Mogadishu, the capital of Somalia.
Somalia has not had a functioning government for 18 years since clan warlords overthrew a brutal socialist dictator then unleashed their militias on each other.
Foreigners rarely travel to Somalia, which is among the most dangerous countries in the world. Kidnappings for ransom have been on the rise in recent years, with journalists and aid workers often targeted. The lawlessness also has allowed piracy to flourish off the coast, making the waterway one of the most dangerous in the world.
