Kim Hak-song, Tony Kim and Kim Dong-chul were moved in early April, the head of a group of families of South Koreans abducted by North Korea told the Yonhap News Agency.
The South Korean news agency said one of the detainees was sentenced to 10 years of hard labor in 2016 for an espionage conviction.
Two others are reportedly scholars with one of them studying accounting, the other one is a specialist in agriculture and taught at a prestigious science and technology university before they were arrested in 2017 on suspicion of "hostile acts."
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The move comes ahead of a planned summit between Trump and Kim in the coming weeks.
Trump's newly appointed National Security Adviser told Fox News on April 29, that North Korea could demonstrate its sincerity and goodwill by freeing American prisoners ahead of the summit.