Two of the victims were teenagers, aged 14 and 16, and the third was a Ukrainian tourist who was not with the group, according to police.
The incident happened at Les Deux Alpes resort, in south-east France, about 50 km from the Italian border.
The teenagers were among a group of 10 experienced young skiers on a high school outing from Lyon with their teacher.
Their teacher had led them onto the black-rated slope that had been closed following several days of heavy snowfall.
At least two other members of the school group were found in a state of cardiac arrest and the teacher suffered multiple injuries.
They were transferred to a hospital in nearby Grenoble.
Public radio station France Bleu said a large amount of unstable snow had fallen on the mountainside at an altitude of 2,500 metres.
Local media reported hundreds of tonnes of snow slid down the mountain during the avalanche.
Some 100 rescuers, dogs and helicopters were mobilised to help in the search for other skiers who might be trapped under the snow.
A spokesperson from the prefect’s office in the Isère region told France Info radio the school group should probably not have been on the closed piste and said an investigation would look into whether warning signs had been correctly in place.