The Western Cape Government Health's Emergency Medical Services (EMS) responded to a suspected gas leak in Diaz Road, Grassy Park at 13:37.
According to EMS rescue personnel on scene, six patients were inside the dwelling.
On arrival, three patients (two children and one adult male) were declared dead on the scene. The remaining three patients, including a child, were taken to Groote Schuur, Victoria and Red Cross War Memorial Children's hospitals respectively.
The South African Police Services, City of Cape Town, Fire and Rescue, together with Western Cape Government EMS, are on scene assessing and monitoring the situation.