It is understood they were sitting outside a pub in a seating area when the accident happened.
A Garda spokesperson said: “A car struck a number of pedestrians on Castle Street, Donegal town, and three persons, a a male and two females aged in their 50s and 60s were removed from the scene to Letterkenny University Hospital.
“Investigations are ongoing.”
One of the women is said to be in a serious condition in hospital.
The collision happened around 8 pm at the junction of Castle Street and The Diamond.
The area is cordoned off, diversions are in place and Gardai remain at the scene.