Well, shoot, the first time I took the test I was confused as you were because the scales weren't that great in measuring people's preferences. Quite frankly, you are the only person who knows which MBTI to belong to.
Anyhow,
The third function (Tertiary function[in this case: Ti or Fi]) is generally understood as the function we consider vulnerable when interacting with others.
As a general statement, INTJs often have trouble with their introverted feeling function and often find it a vulnerability to themselves. Thus, when the Fi function hasn't mature, it can take a nasty turn with themselves or with others.
INFJ's, with their Introverted Thinking on the other hand, often over-think(sorry for the pun) things. Say the INFJ will be doing a paper that is stressing the said INFJ out. That INFJ will work on and on with that paper until it fits to how the INFJ wants it to be, or snap out of it and accept it as is.
NOTE: And fudge, I gave you the wrong link
http://personalitycafe.com/cognitive-functions/9813-mbti-functions-explained.html