Thermistor motor protection relay for thermal protection
From several sources, I have found that IEC 34-11-2 provides specifications to provide sensor and protective relay interchangeability. The relays are intended to trip based on resistance measurement, not voltage. The measurement voltage must not exceed 2.5 volts to avoid self heating of the thermistor. The diagram below shows the characteristics that the thermistor must meet to be in conformance with IEC 34-11-2.

Normally three thermistors are connected in series. The sum of the resistances of the three thermistors plus the resistance of the external connecting wire must meet the requirements shown below to be compatible with a protective relay that is designed to work with a sensor that meets IEC 34-11-2. You have not stated if the motor supplier says the thermistors meet IEC 34-11-2 or what the resistance vs. temperature characteristics are.