They found residents' festering bed sores were not treated in one centre while in another property, residents in acute pain were denied medicines. In one of the privately-run homes, faeces was found on a tablecloth.
Staff in all three were under-trained in even the basics of care for older people, such as how to treat people with dementia, the Care Inspectorate said.
The homes were given the lowest possible grading on every aspect of care.
The Livingston Nursing Home in West Lothian, Highview House Care Home in Inverness, and Avondale Nursing Home in Motherwell, all received a grading of one on a scale of one to six and were described as unsatisfactory.
The facilities were identified this week by the inspectorate as the current three worst preforming homes in Scotland. The results prompted anger from politicians who said families deserve better.
Two staff at the Livingston home have been charged by police over alleged standards, and care inspectors criticised performance in areas ranging from continence management to cleanliness.
At Avondale, inspectors wit-nessed unsafe moving procedures, found "overpowering" odours from clinical waste bins and said the kitchen was not fit for purpose.
The inspectorate backed 21 complaints in the last two years about Highview. The report said one resident waited 90 minutes for a nurse after calling for help.
Inspectors visit all 897 homes in Scotland at least once a year but conduct repeated visits at the poorest performing facilities.
The inspectorate said Highview House Care Home had had an improvement notice and its suspension on new admissions lifted, but a ban on new residents remains in Livingston and Motherwell. All three face closure if they fail to improve.
Highview is expected to improve when the next round of reports are published. The grading system for inspections can change through the year as they are carried out.
Annette Bruton, chief executive of the Care Inspectorate, said: "Most care homes and nurseries in Scotland perform well, with less than 1% falling into the lowest grade. Those services must improve fast or face closure because every person has the right to high-quality, safe and compassionate care.
"Our system of robust, unannounced inspections means problems can be picked up early and the right support put in."
Jackson Carlaw, health spokesman for the Scottish Conservatives, said: "It is completely unacceptable for care homes to be registering such appalling results. How a facility can be judged unsatisfactory in every area is astonishing, and when it happens severe and immediate action must be taken."
Rhoda Grant, Scottish Labour's wellbeing spokeswoman, said the results were "disgraceful."
A spokesman for Four Seasons Healthcare, which owns the Livingston home, said an improvement plan had been put in place and "all residents have been offered the opportunity to transfer to a different care environment but have declined".
A spokeswoman for Avondale said the inspectorate's findings were "taken very seriously" and it was "confident the home is making good progress".
Highview took "immediate action" and a spokeswoman said it had had "a verbal confirmation of the improvement of its next grades".
Scottish Care, which represents independent social care services in Scotland, said failures in standards are of "deep concern".