It happened inside the Daisy cocktail bar on Queen Street at Portland Street around 2:45am
Paramedics say two male victims suffered serious but non-life-threatening injuries, while a female victim was taken to hospital with minor injuries.
A fourth victim also suffered minor injuries as a result of the stampede to get out of the building.
A witness who spoke to CP24 at the scene said he heard a barrage of gunfire and soon saw dozens of people fleeing the building.
“It was like an earthquake.  All of a sudden, like at the door they gathered and then they were running left and right, back and forth,” said the man.  “A lot of people were confused and didn’t know what to do.”
There is currently no information on possible suspects.
Queen Street West was closed from Augusta to Portland streets for several hours for the investigation, but has since reopened.
Police are appealing for any witnesses with information to come forward.
The shooting comes after a violent weekend in the city in which several people were shot, including a 15-year-old boy who was murdered in North York. 
On Sunday, Mayor John Tory issued a statement about the rash of violence, calling it “extremely troubling”.