The badly burned body of the male teenager - whose identity has yet to be confirmed - was found after firemen tackled a blaze in a shed on allotments off Newcastle Road, Sunderland, Tyne and Wear, on Friday night.
The grim find was made within yards of a shed where firefighters found the body of Thomas Kelly, 18, in November.
And fewer than three weeks ago, David Hanson, 15, was found dead within half-a-mile of the allotment site, in a burning derelict former guest house in Roker Terrace, Sunderland.
David, of St Peter's View, Sunderland, was a pupil at the town's Monkwearmouth School.
The latest victim has not yet been identified with certainty, but is believed to have been one of David's classmates.
Detective Chief Superintendent Barry Stewart, head of Northumbria CID, told a news conference he was satisfied none of them died as a result of foul play.
He said that although they were not homicides in the conventional sense, they were sufficiently suspicious to cause some concern. 'We are appealing for any assistance possible from members of the public,' he said.
'I think there may be an explanation which is related to drugs, solvent abuse, experimentation with fire and other methods that will no doubt be more apparent as the investigation continues.'
'If it is a new craze, it has got to stop,' Mr Stewart said. 'We have three deaths and we have to ensure that those who are experimenting stop.
'We need to know the answer as quickly as we can otherwise there could be more deaths.'