The three, who were not injured, were suffering from shock.
Police, the fire service, paramedics, and utilities engineers are working to discover the cause of the blast at the two-storey building in the Chapel Street area of Bentley, near Doncaster, at 6.55am.
Residents of 14 ground and first floor flats were evacuated from the building and are being cared for at a nearby pub.
A team of engineers will assess the structural integrity of the building before deciding whether they can return or will require council accommodation.
A South Yorkshire Police spokesman said: "There was a gas explosion at the block of flats.
"All the occupants have been evacuated from the building for their safety.
"Thankfully there are no reports of serious injuries and three persons have been conveyed to Doncaster Royal Infirmary suffering the effects of shock.
"Other residents of the flats have been evacuated and are being cared for at a nearby public house whilst utility services at the scene work to identify and rectify the cause.
"The fire service, gas and electricity board are all on the scene and everything is in hand."