The condition the three children were suffering from meant they were unable to talk or walk and were slowly dying.
It has been described as an incredibly debilitating disorder.
The four-year-old girl was diagnosed first followed by the younger twin boys.
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The 20-year-old was found fatally wounded on Roth Walk, near Finsbury Park, north London at around 3am on Saturday, Scotland Yard said.
Police have arrested a man wanted in connection with a burglary at the home of pensioner Richard Osborn-Brooks.
The rally comes after it emerged that the Home Office destroyed thousands of landing cards documenting the arrival of windrush-era migrants