Miranda Harris, 66, and married couple Chris Naylor, 58, and Susanna Naylor, 54, died when their car came off Swarkskop Bridge near Port Elizabeth, Eastern Cape Province.
Mrs Harris's husband Peter, 67, survived the crash with injuries, alongside the driver of the car.
Mr Harris is in a stable condition in a Port Elizabeth hospital.
It is thought that the car went off the bridge after it collided with another vehicle on the N2 motorway in Bluewater Bay, on South Africa's southern coast.
The two couples were in the country with Christian charity A Rocha, founded by Mr and Mrs Harris in the early 1980s.
Mr Naylor, executive director of A Rocha International, co-founded A Rocha Lebanon in 1996 with his wife, having both worked as teachers in the Middle East.
In recent years, Mrs Naylor was head of science at Christ Church Cathedral School, Oxford.
Rev Dave Bookless, director of theology at A Rocha, said: "The death of Chris, Miranda and Susanna leaves a huge and irreplaceable hole in our lives and our organisation.
"Our prayers are for Peter's full recovery, along with the driver of the vehicle they were travelling in, and also for the Harris and Naylor families, as well as the hundreds, if not thousands, who consider them as extended family.
"Now is a time for grief, and sharing our treasured memories of some godly people."