The two men and a woman were admitted to Glasgow's Western Infirmary, suffering from the effects of smoke inhalation, following the blaze at the property on Shiskine Place, Maryhill.
Another woman was also treated at the scene but did not need to go to hospital.
The flames took hold in the ground floor in the early hours of Saturday, but details were only confirmed by the Scottish Fire and Rescue Service (SFRS) today.
Around 25 firefighters attended the incident, which took place around 3:50am.
The fire service said they rescued two of the three people who were taken to hospital. The woman and one of the men, who are believed to be a couple, were rescued from the first floor of the building by fire fighters using a ladder. They were taken to the Western along with a man from a ground floor property.
None of the casualties' injuries are thought to be life-threatening and the incident is not being treated as suspicious.
Group Commander Iain Goodlet, the senior officer at the scene, said: "Four people sustained smoke inhalation and the quick response of firefighters and ambulance crews ensured they got to safety and received immediate medical care."