Croatian nationals Renato Garma and Mate Lusa and Montenegrin Milan Smolovic, working for the oil services company Hydrodive, were kidnapped on February 19 from a bar outside their base in the city of Port Harcourt, with the kidnappers demanding a ransom.
Employees of foreign oil companies in Nigeria, the world's sixth largest oil producer, are frequently targeted by armed groups which use kidnapping to achieve political goals or seek ransom.
The news of the release of the three hostages was confirmed early on Monday morning by the Croatian Foreign Affairs and European Integration Ministry.
"They were released at 5:30 hours on Monday and are now in hospital for medical check-ups," Ministry spokesman Zeljko Belaj said.
A community leader in Ogbakiri, a waterside village a few miles outside Port Harcourt, said troops had invaded the area shooting, made several arrests and beaten up residents, Reuters reported.
"They said they were looking for the people who kidnapped some expatriates. They were shooting and everyone ran into the bush. Some people were wounded and some arrested," the community leader said, asking not to be named because he is being monitored by the security agencies.
"The community is innocent. They want the government to bring water, electricity and a hospital," he added.
Attacks by armed militants on foreign oil companies and their workers have been reported in the Niger Delta since January this year.
Violent protests against foreign oil companies are frequent.
Around one hundred people were kidnapped in the Niger Delta area over the past year, and most of them were released after the ransom was paid. Most kidnapped people worked for foreign oil companies.
Most oil companies are insured against kidnapping and pay ransom in most cases.