This is a really tough and nuanced question to answer, oh boy.
This is going to largely be influenced by my perspective as a Stark player predominantly. Generally speaking, each faction wants 2 zones that either synergize with the play style of the combat units on the board (Baratheons might want the bags before they take the swords to make their tough units last longer, for example, or certain units getting to proc different effects than the zone would normally allow such as "Onslaught") or, as tends to be the case more frequently, their cards encourage the possession of those zones such as Starks' Devastating Impact having a stronger effect when the Stark player owns the Horses space.
So, that tends to mean that (on top of wanting to claim the zones' effects themselves), you'll always want at least 2 NCUs to claim the spaces you need and have some flexibility in claiming zones beneficial to the state of the game independent of card effects.
But the choice to go from 2 to 3 NCUs is an interesting one that depends on a lot of different factors. It might be that you want an unkillable activation that's cheaper than a combat drop, or that the NCU in question's ability is a nasty one, or maybe you just have an awkward 3 point gap in your list that's not useful for anything else. Generally speaking, though, I think most players would ascribe to the maxim that 2 NCUs covers your faction's tactics board needs for your cards, and the 3rd is purely for area denial or situational tactics board needs (say needing the money bags after you just got slammed by some Mountain's Men).
From a Stark perspective, if I'm taking Howland Reed I always have 3 NCUs by virtue of having a free one. Generally, though, since most of the Stark NCUs have actually rather potent Influence abilities that mesh well with the faction, I tend to take 2 of them for 7-8 points because I'm taking them just to cover my bases and provide my army with an extra punch. If I have the points leftover, I take Sansa or Arya for the once-per-game boost and cheap activation + zone claim (unpopular I know, since Sansa is usually the starting point for all Stark lists).
My usual Stark NCU suite varies depending on the list. My tough, defensive Tully list runs Ned Stark and Sansa. My Howland lists run Howland, Arya, and Sansa/Catelyn. And my Robb lists usually have Rodrik and Arya/Catelyn. If I think I'll be facing a particularly nasty NCU suite or a list built to dominate the tactics board, I'll try to stick to just 2 NCUs, but it's really difficult to peg exactly why any army chooses 2 or 3.