The group got into difficulties while swimming in the lake at Pulpit Rock, south of Ardlui, on Saturday evening.
A 41-year-old man, a 29-year-old woman and a nine-year-old boy were pronounced dead at the scene. They have not yet been named.
A seven-year-old boy was rushed by ambulance to the Royal Hospital for Children in Glasgow, where he is currently in intensive care
A Police Scotland statement said it received the call at about 6.40pm on Saturday.
It added: “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 of a 41-year-old man, 29-year-old woman and nine-year-old boy have been made aware.”
A report into the incident will be sent to the procurator fiscal.
It brings the number of people who have died in Scotland after getting into difficulty in the water to five in just 24 hours.
A 16-year-old boy died in the water at Balloch Country Park, at the south end of Loch Lomond, on Friday.