oil change warning message coming on but oil change done 3,000 km ago??? Could the shop have reset the system incorrectly?

It is indeed possible that the minder for this service may not have been reset properly. However, for Diesel engines especially, "engine hours" are also important. If the engine is often running without logging distance, it still will need to be changed regardless of distance traveled. An example would be a Diesel powered vehicle that idles constantly at a construction job site while work is being performed, or even driven by a power take off unit (PTO) from the transmission to some kind of accessory. In any case, if this is a personal vehicle used for simple "daily driving" it could be erroneous. Seek guidance locally.

It's very possible they didn't reset the light. If you know for sure how many miles you have gone then stick with that and change at the normal interval you have been using. Now with GM they do use an oil life monitoring system that takes load, temperature, engine rpm and few other things into account. So technically if you were really hard on the truck, maybe towing for a long time it could cause the light to come on a little earlier but 3k km is very early. Alex