A 41-year-old man, 29-year-old woman and nine-year-old boy died at the scene near Pulpit Rock, Ardlui in Argyll and Bute on Saturday evening.
A seven-year-old boy was rescued and taken to hospital in Glasgow where he is now in intensive care.
There have been five water deaths in two days in Scotland, with incidents at Loch Lomond and South Lanarkshire.
Police said they were alerted at about 6.40pm on Saturday following “reports of concern” for people in the water.
A Police Scotland spokesperson said: “A multi-agency operation took place and sadly, three people were pronounced dead at the scene.
“Formal identification has still to take place but the family have been made aware.
“A seven-year-old boy was taken by ambulance to the Royal Hospital for Children in Glasgow.”
Scottish Fire and Rescue Service said appliances from Killin, Tyndrum and Balloch were sent to the scene, along with water rescue units from Oban and Stirling.