This is in the Tongariro National Park, and the three mountains would be Ruapehu (the North Island's highest), Tongariro itself, and Ngauruhoe. Ngauruhoe and Ruapehu are considered active volcanoes.
You probably mean the three volcanoes that are together in the centre of the North island. Their names are, Mount Ruapehu, Mount Ngauruhoe and Mount Tongariro.