The vehicles, which cost Â£400,000, have been bought by South East Coast Ambulance Service (SECAmb).
The new ambulances have wider lifting equipment and specialist handling aids.
SECAmb operations director James Pavey said the ambulances were not exclusive to bariatric patients.
He said: "These vehicles can be used for any patient, from the outside you wouldn't notice them to look much different from the average ambulance."
Mr Pavey said methods of moving obese patients without specialist equipment were "not particularly dignified or humane".
"We have a number of increasing issues with people who are more than morbidly obese and weigh up to 50 stone.
"We will probably need to increase the size of the fleet to meet the demand but this is the first step."
The specialist equipment includes a Megasus stretcher, gantry and mobile hoists, threshold ramps and enhanced air cushion lifting aids as well as a stair-climber chair.
When Mr Pavey was asked if these new ambulances meant the NHS was giving up on trying to get obese people to lose weight, he said: "We're responding to the needs of patients who are acutely unwell.
"Chronic conditions turn into acute conditions very quickly because of the complications of their weight. We're by no means giving up as far as the NHS goes."
The vehicles will be based in Paddock Wood in Kent, Chertsey in Surrey and Worthing in West Sussex.
Three bariatric ambulances are coming into service in the south east to cope with "increasing issues" with patients who weight up to 50st.