Two men died at the scene, and a young woman died later whilst being taken to hospital.
The trio - a father, his daughter, and her husband - had gone out on Sunday evening to look for mushrooms.
The 54-year-old man became trapped in the eight metre deep shaft, in which poisonous gases had formed.
The younger man went to alert the ambulance and fire service, and the 26-year-old woman tried to save her father but is believed to have fainted in the shaft because of the poisonous gas.
The 32-year-old son-in-law then tried to save the father and daughter, but also became faint and dizzy.
By the time the fire brigade arrived all three were trapped in the shaft and unconscious. They were given breathing apparatus but both men died. The woman was flown to hospital by helicopter but died en route. It is thought that she may have been pregnant.
Firefighters found high levels of carbon monoxide gas in the shaft.
The site of the accident is a few hundred metres from an agricultural estate which the deceased family owned, according to Manfred Ehrgott of the Red Cross.
It’s not clear if the 54-year-old man fell down the well shaft, or had climbed down on purpose to check something. The shaft is in a wooded area, and had only recently been dug. An autopsy will determine the exact cause of death, which is believed to be carbon monoxide poisoning.