Exhaust 3 temp fault code

[JMBT]

Well temp 1 would be the DOC temp sensor, 2 would be the DPF inlet temp sensor, so 3 would be the DPF outlet temp sensor.
I would say either the sensor is bad, or the DPF is full of ash, or possibly cracked. Could also be, although very doubtful, the temp sensor hub unit that all three plug into. it sets on top of the DPF can and you will see all the temp sensor wires going to it. You can test it with a multimeter from pin to pin to make sure none of the circuits are grounding out inside. Quickserve has the testing instructions.
Next:
Do you have INSITE?? If so connect and see what the #3 sensor shows compared to the other temp sensors after the truck has been off overnight and completely cooled down. They should all be relatively the same give or take a degree or two. If it is way off from the others replace the sensor. If it is nearly the same as the others pull and inspect the DPF and DOC.
Good luck hope this helps