Two teenagers are fighting for life after being stabbed outside an inner-city Sydney hotel where they had been celebrating a birthday.

Police are investigating the incident, which occurred on Cleveland Street, Redfern, about 4am on Sunday and left three young men injured.

The three men, all from Western Sydney, had been at a 22nd birthday party at the venue, a booking of up to 300 people.

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Once it finished, police were called to move on the group, which spilled onto nearby streets about 3am.

Officers said they then found an 18-year-old unconscious and bleeding heavily after being stabbed in the stomach, a 19-year-old also critical with a stab wound to the chest and a 20-year-old with a wound to his arm.

Inspector Dominic Wong from NSW Ambulance said paramedics arriving at the scene had "great concern for all three patients, particularly the teenager who was unconscious and bleeding heavily from his abdomen."

Wong said all three men "injured were known to each other".

"It is concerning that people would attend a party armed with a knife," he said.

"We don't know where the knife has come from, we haven't established who is an offender."

Investigations are ongoing.