Why does a person seek external validation?
Applying this to an Enneagram type, the 3 stands out most to me:
How much pride do you actually feel your achievements?/How important are your achievements to YOU?
Do you dwell on your achievements, or do you cast them aside and move on?
How easy do you feel your achievements are?
Theorizing about 3s, I don't think they have an internal complimentor. They find their accomplishments easy, because they have done them. The external validation allows them to realize that what they are doing, are actually worth two-shits. They have an internal critic, but not a strong positive force within.
Constantly seeking an identity.
The potential and drive they have, may actually stem from a "hollow" core. Threes accomplish a lot, because they are never satisfied within themselves.
By far, Threes are the most out-of-touch with their emotions in the "heart/feeling/image" triad. If you don't feel you're a solid person, how can that allow one to feel? In the quest for an identity they will compartmentalize emotions for "later", they don't really know themselves yet.
Ironically, emotions get in the way of a three identifying themselves. I find that a 3 takes a long time to mature, they are looking outside when they should be looking inward.
It's rare for someone to feel that they know a 3, always behind a mask.