Your third detachment must not be CAD like. Meaning that if the detachment that you want to take has all 5 choices (HQ, Troop, Elite, Fast Attack, Heavy Support) that is CAD like and not allowed in the third slot. Examples of CAD like detachments are Company of the Great Wolf Detachment (from Champions of Fenris Supplement), Real Space Raiders Detachment (from the Dark Eldar Codex) and Nemesis Strike Force (from the Grey Knights Codex). Now if your detachment doesn't have all 5 choices (HQ, Troop, Elite, Fast Attack, Heavy Support) it is a non-CAD like detachment. Examples of those are Masque Detachment (from the Harlequin Codex), Skitarii Maniple (from the Skitarii Codex) and Officio Assassinorum Detachment (from the Officio Assassinorum Dataslate).
Also remember with the release of the new Imperial Knights codex, they have essentially been banned from the NOVA Open format and will be treated the same way at the RTT this weekend.
If you have any questions please let me know. You can reach me at elphilo40k at gmail dot com.
I'll see you Saturday!