Emergency services and the gas board were called to a flat in Garvel Road in Glasgow's Barlanark area at 2.15pm yesterday.
Two crews from Easterhouse Fire Station scrambled to the scene where they cleared people out of the four-storey flat.
"Firefighters forced entry into the flat and evacuated the building," said a spokesman.
Two menand one woman were taken to Glasgow Royal Infirmary with suspected carbon monoxide poisoning.
A spokeswoman for Scotland Gas Networks said: "We were called to the flat following reports of a gas escape. Our engineers have now left the property."