Thames Valley Police is appealing for the public's help to investigate how the crash happened.
The crash was referred to the Independent Office for Police Conduct (IOPC), who will decide on the mode of investigation in due course.
Inspector Gavin Biggs, of the Roads Policing Unit, said: “I am appealing for witnesses to this three car collision on the A340 near to Tidmarsh.
“We are conducting an investigation into the circumstances of this incident and would seek help from the public to aid our enquiries.
“I would ask that any motorists who were using the road at the same time, particularly anyone with dash-cam, get in contact with us if they can help in any way.