While the research field of hyperthermia has long focused on magnetic oxides and two-component alloy nanostructures, ternary magnetic alloy nanoparticles (NPs) have not been thoroughly investigated and understood so far. In this study, we synthesized FeNiCo ternary alloy NPs with variable compositions by a facile polyol method. By investigating morphological and magnetic properties using field-emission scanning electron microscopy, hysteresis loop measurements and first-order reversal curve analysis, we found average NP size, saturation magnetization, coercivity (HC) and coercive field distribution (ΔHc) to range between ~60–230 nm, 65–109 emu/g, 33–98 Oe and 0–650 Oe, respectively. Hyperthermia measurements were performed under an alternating magnetic field with intensity of 400 Oe at a frequency of 400 kHz, leading to maximum specific loss power (SLP) values ranging from 237 to 298 W/g in an ethylene glycol medium with concentrations of 5–1 mg/ml, respectively. By comparing between results of average NP size, HC and ΔHc, our study indicates the possibility of tuning SLP by morphological and magnetic characteristics for potential hyperthermia applications.