We describe a new mK-range nanoscale thermometer, based on a unique three-junction radio frequency single-electron transistor. The three-junction geometry allows separation of the thermal and electronic pathways, providing a potentially significant reduction of measurement-induced Joule heating. A radio frequency embedding tank circuit allows very fast readout. We demonstrate electronic and thermal operation, supported by numerical simulations. Applications to minimal back-action calorimetry and bolometry are discussed.