The plane took off from Sion airport just before 1pm, Valais police said.
However just 15 minutes after take-off it crashed above Savièse on the Sanetsch pass at 2,100m altitude.
Firefighters from Sion airport and three Air Glaciers helicopters attended the scene, however the plane's three occupants were pronounced dead.
The victims have not been formally identified.
The reasons for the crash have not yet been determined, however guests at a hotel on the Sanetsch pass told Valais paper Le Nouvelliste that they saw the plane flying at very low altitude.
The crash is the latest of several in the past few weeks.
On August 4th two teenagers and a pilot were killed when their plane crashed near Diavolezza in Graubunden. The flight was meant to be the highlight of a summer camp where youngsters had been learning about aviation.
And on August 8th two people died when their single-engine plane crashed into Lake Constance on its way from Zurich to Hamburg.