Police were alerted at around 6.15pm after a member of the public discovered the bodies in the area, which is between Barton Stacey and Chilbolton.
The bodies have not been formally identified but they are believed to be; Michael Pedersen, aged 51 of Chertsey, Surrey; Ben Pedersen, aged 7, and Freya Pedersen aged 6, both of Ashford, Middlesex.
The Coroner has been informed and post mortem examinations are due to take place.
Detective Superintendent Tony Harris, said: “We are currently investigating the circumstances surrounding this terrible incident and are working to establish what has led to these tragic deaths.
“We are treating this as an isolated incident and we do not believe anyone else was involved.
“The families of Michael, Ben and Freya have been informed and specialist Family Liaison Officers from Hampshire Constabulary, and officers from Surrey Police, are working with them to provide support and keep them updated as the investigation progresses.
“Our thoughts are with the family and friends of those involved at this extremely difficult time and I would ask that they be allowed to come to terms with what has happened in a dignified and respectful way.”