One of those men, whose age is unknown and who is yet to be identified, died at the scene.
The other, in his 40s, is being treated by critical care medical staff, after being unconscious and underwater for three minutes.
He is in a critical condition and has been taken to St George Hospital in an Ambulance.
The third man, aged in his 20s, is conscious and has been taken to St George Hospital in an ambulance for further treatment.
It is also believed that a rescuer is being treated for exhaustion after swallowing water.
Sutherland Shire Police are investigating the incident, and are expected to prepare a formal report soon.