I assume you have the version of Azteeg X3 with thermocouple inputs (they're optional). Can you find out what thermocouple amplifier there is on the X3? It's on the shield. I can't seem to find schematics for it anywhere. I found an
X3 schematic here, but it doesn't have the shield (so no thermocouple amplifier, no extra thermistor inputs). Also, I don't know which input pin the thermocouple inputs would connect to -- you need to know that to configure Marlin correctly (my guess would be one of the pins on the EXP2 connector -- A4, A5, A11 or A12). Note that you can't connect a thermocouple to a thermistor input -- you have to use a proper thermocouple input, with an amplifier. Also note that it matters which way you wire the thermocouple. When heated, it generates a tiny DC voltage, which the circuit measures, so it needs to be plugged in the right way round. This may help with testing: with the thermocouple disconnected and the input pins shorted, the firmware should read room temperature (assuming a compensated amplifier, which is usually the case).