As far as I am aware this doesn't exist. There is a telnet guide, but draytek obviously don't want to maintain it (every router has slightly different commands). It simply doesn't apply for the 3900.

I was a bit happier when I worked out that the bottom half of the help screen is contextual (changed depending on which command you are in).

once you log in to telnet (if lets you) by typing help should show available options