A pod of 11 common dolphins beached at the Godrevy headland, but two off-duty firefighters managed to get eight back in the water.
Two visitors spotted them around 7am on Monday morning, 17 September.
Dan Jarvis, from the British Divers Marine Life Rescue, said it was "really unusual" for a large number of dolphins to be stranded simultaneously.
Mr Jarvis said the firefighters had "done a fantastic job" in floating eight of the dolphins back into the sea.
Unfortunately, the remaining three had already died before the rescuers arrived on the scene. It is believed that they had been stranded the longest.
Mr Jarvis said that there had not been a mass stranding since the common dolphin mass stranding in the Fal estuary in 2008..
20 Cornwall Wildlife Trust and British Divers Marine Life Rescue volunteers are monitoring there and nearby beaches, hoping they don't return.
The volunteers will conduct post-mortems on the dolphins to try to figure out what happened to them. They say one theory could be the dolphins came to the beach to try to help one of them who was sick.
We are proud of our local volunteers for stepping up to the mark, as usual, when required. equally, it is sad that three of the animals died.