The CareFlight medical rescue service said the barbecue exploded at a home in Glenfield on Christmas Day afternoon.
Police, fire, ambulance and CareFlight crews rushed to the home, the rescue helicopter landing in a field 200 metres from the house to allow the crew's trauma doctor to assist ambulance paramedics treating the three.
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The CareFlight doctor said the barbecue erupted into a fireball when what is believed to be methylated spirits was used to help start it.
A seven-year-old boy and his mother, from Kellyville in Sydney's northwest, suffered leg burns, as did another woman.
The three were taken to hospital by road ambulance.
