This one's pretty easy to answer. 1.d4 Nf6 2.c4 g6 3.Nc3 is far better than 3.Nf3 for most players. 3.Nc3 allows white to choose between the Saemisch with f3, classical lines with Nf3, and the sharp Four Pawn Attack with f4.
The drawbacks of 3.Nc3 are almost non-existent. If white plays 3.Nf3, he only limits himself, not his opponent.
I want to be absolutely clear: I don't believe 3.Nf3 is actually an objective mistake since White can transpose into mainlines fairly easily. But there is almost no good reason to play 3.Nf3 over 3.Nc3.
The only line where 3.Nf3 is preferred over 3.Nc3 is the fianchetto variation. And even there, play almost always transposes. If you really want to play the fianchetto lines, you're probably best off playing 3.g3 first! That's Avrukh's recommendation, based on what he plays against the Grunfeld rather than the KID.
When your opponent plays the King's Indian Defense, do you go for 3. Nc3 or Nf3? Or another move altogether? What are the positional advantages of each? I personally go 3. Nc3 because I love the four pawn attack... It leads to fun games (for me). But I don't know what's actually positionally better.