According to the Scotland Yard, the youth included two boys aged 17 from northwest London and one man aged 19, Xinhua reported.
They were reported missing on Friday and were detained in Istanbul after Britain alerted Turkish officials.
“Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria. They remain in detention in Turkey,” a British police spokesman said.
Turkish authorities were in touch with their Britain counterparts to arrange their deportation to Britain this week, reports said.
The families have been kept informed of developments, Scotland Yard said.
In the end of February, three British girls were reported missing and crossed into Syria.
The girls–Shamima Begum, Amira Abase, both 15, and Kadiza Sultana, 16, are students from the Bethnal Green Academy, a school in east London. They were believed to cross into Syria and joined the IS in February, police said.