I've worked on D/3 systems on and off for 25 years. Unlike other automation vendors, like Rockwell, you're likely to find few instructional materials online for the D/3. Though D/3 ownership has changed hands many times over the years, the policy of closely guarding manuals (partly to protect their ed services business) persists.
Your best bet is to spend time studying some of the plant's applications alongside their licensed copies of electronic manuals (or older bound manuals, if you can find them). Alternatively, Novatech offers courses in all aspects of D/3.
Good luck with the new job!