Crews from Codsall and Wombourne were called to Duck Lane, Bilbrook in Codsall just before 6pm on Saturday following reports of a carbon monoxide alarm activating.
"This incident highlights how essential carbon monoxide alarms are as carbon monoxide is colourless, tasteless and odourless- it is a silent killer."
Codsall Watch Manager, Dave Bloomer
On arrival, the two elderly occupants and their daughter, who was visiting, complained of feeling unwell and had headaches. They were assessed by paramedics and were all found to be suffering from high levels of carbon monoxide.
Firefighters used a gas monitor to test the property and found high levels of carbon monoxide were present.
The casualties were transported to hospital for further assessments whilst firefighters ventilated the property and made the boiler safe, as it is believed this was the cause of the leak.
Codsall Watch Manager, Dave Bloomer said: "We have specialist equipment at each fire station for incidents involving gas and were able to immediately confirm that the carbon monoxide alarm at the property was operating accurately.
"Quite high levels of carbon monoxide were found to be present inside the property and paramedics confirmed all three people has tested positive for carbon monoxide poisoning.
"This incident highlights how essential carbon monoxide alarms are as carbon monoxide is colourless, tasteless and odourless- it is a silent killer.
"We actually visited this property six months ago and carried out a free Home Fire Risk Check, which on that occasion included supplying a carbon monoxide alarm as well as a smoke alarm.
"Had we not have fitted that carbon monoxide alarm, the occupants may not have been as lucky as they have been as it does not take long for the gas to fatally poison people."
The incident was closed at 6.45pm.