If the other components in your system produce so much heat I think it will be very complex, if not impossible, to calibrate the software to compensate for this. The measured temperature is not only a function of the environment temperature but also air flow over the triacs, direction of the airflow, thermal isolation (i.e. dust) and miscellaneous other factors.
The only realistic option is to increase the distance to the triacs, to prevent them from influencing the thermistor.