Second/Third Generation Death Knights

Is it at all possible to have a third gen death knight who has been undead for more than three years?
If not, what are the limits to a second gen death knight?

It is possible. Forsaken Death Knights are essentially 3rd gen Death Knights who were already dead. They just got re-reanimated, or re-bound under Lich Kings will.

Also a possibility of a Necromancer of any race who was already undead when was caught by Lich King & Scourge and Death Knight'ed.

I don't know how far they address your questions specifically, but I found both of these a good read:

http://eu.battle.net/wow/en/forum/topic/2504881097
http://wow.joystiq.com/2007/08/09/know-your-lore-death-knights/

I would just not use the Gen system, its made up and misses out on a lot of detail.

But to answer your question... I dont think you could have been made in the acherus before it was active no.

However, Death knights who were killed and raised and can swap weapons were around pre Acherus. Thassarian and Mograine for example.

WOOHOO

PM me for any DK questions, be happy to help.