Shamima Begum, 16, Kadiza Sultana, 17, and Amira Abase, 16, all of whom attended Bethnal Green Academy in east London, ran away from home in February last year to join another school friend who had left in December 2014.
All of them were married off to men approved by the terrorist group to become so-called "jihadi brides", with two becoming widows within months of arriving in Syria, their families believe.
"The last message from the girls was a very short one, saying that bombs were going off fairly close and that communications were likely to be disrupted," she said.
In communications with their families in London, the girls say that IS has banned the use of mobile phones in the stronghold of Raqqa.
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One of the three teenagers had reportedly been in regular contact with her parents until recently.
Their last communication with their families in the UK came in mid-December 2015 and all said they were living in Raqqa, which is now the focus of a US-led bombing campaign.
In March last year Scotland Yard commissioner Bernard Hogan-Howe had said the teenagers could return home without fear of being prosecuted for terrorism, as long as no evidence emerged of them being engaged in violence.
Their families had hoped they may return but the hope is now fading.