Howdy Serge,
It looks like I may have had my wires crossed. Just consider it as a post with addition information :). Regarding the AMC7823 device, if you are referring to the internal temperature sensor, the datasheet does specify an accuracy of +-2C for a Vref voltage of 1.25V, and an accuracy value of +-4C for a Vref voltage of 2.5V. The accuracy values seem to correlate with the number you have given, +-3C.
An illustration of the relationship between the internal reference and temperature is shown in the figure below. The temperature affects the reference voltage which drives the systems resolution and accuracy.
When using the internal reference, a filter capacitor (10uF) connected from pin 21 (EXT_REF_IN) to GND is recommended to improve temperature reading.
If you are referring to the precision current source, then more information is needed, such as a copy of the thermistor or RTD circuit, Rset value, and reference configuration (internal/external).
Hope this information helps.
Best Regards,
Matt