user18829: After:
[FEN ""]

1. e4 g6 2. d4 Bg7

by far the most common third move is 3. Nc3. This goes against the principle of not blocking the c pawn with the knight as white, now of course principles can be broken but I can't see the benefit of doing so here.
The e4 pawn does not need defending, and if the aim is to stop 3... d5 then 3. c4 achieves this as well, then Nc3 can be played later.
So why is it not preferable to play 3. c4 before Nc3?

user12873: 3.Nc3 saves a tempo by not pushing the c-pawn first, which could be important in some sharp lines. However, 3.c4 is equally as good, transposing from the Pirc into the King's Indian.
It really just depends on your preferences.

user18809: There is no such principle as "don't block your c-pawn". A c3/c4 pawn push is better in some positions and worse in others. What White intends is to develop his queenside fast, castle there and launch a kingside attack with his pawns, so c4 is out of the question if you want to play that plan.
Indeed, White would want to do the same thing against the King's Indian, the reason why he doesn't is that Black would not allow it! See for instance 1.d4 Nf6 and now 2.Nc3 is not done because 2...d5 would come, otherwise, if would be the best move!

user14202: It is just a matter of taste.  White is playing a king pawn opening so with the pawn on e4 the principal of blocking the c-pawn does not apply.   White simply doesn't want to play a King's Indian defense.
With Nc3 White has a lot of options:  

For one he can seek a more Dragon type game with the idea of Bc1-e3, 
Qd2, and h2-h4; often with f2-f3 being played.
Alternatively, White    can go for f2-f4 lines probably pushing
e4-e5.
Or White can play    classically with Nf3 and Be2.

So overall a flexible developing move.

user9321:  3. c4 will likely transpose to a King Indian defense while 3. Nc3 will likely lead to a Pirc. 1. e4 players will prefer the more familiar Pirc positions.
The Modern Defense move order lets White decide the main position.

18829: <p>After:</p>

<pre><code>[FEN ""]

1. e4 g6 2. d4 Bg7
</code></pre>

<p>by far the most common third move is 3. Nc3. This goes against the principle of not blocking the c pawn with the knight as white, now of course principles can be broken but I can't see the benefit of doing so here.</p>

<p>The e4 pawn does not need defending, and if the aim is to stop 3... d5 then 3. c4 achieves this as well, then Nc3 can be played later.</p>

<p>So why is it not preferable to play 3. c4 before Nc3?</p>
 12873: <p>3.Nc3 saves a tempo by not pushing the c-pawn first, which could be important in some sharp lines. However, 3.c4 is equally as good, transposing from the Pirc into the King's Indian.</p>

<p>It really just depends on your preferences.</p>
 18809: <p>There is no such principle as "don't block your c-pawn". A c3/c4 pawn push is better in some positions and worse in others. What White intends is to develop his queenside fast, castle there and launch a kingside attack with his pawns, so c4 is out of the question if you want to play that plan.</p>

<p>Indeed, White would want to do the same thing against the King's Indian, the reason why he doesn't is that Black would not allow it! See for instance 1.d4 Nf6 and now 2.Nc3 is not done because 2...d5 would come, otherwise, if would be the best move!</p>
 14202: <p>It is just a matter of taste.  White is playing a king pawn opening so with the pawn on e4 the principal of blocking the c-pawn does not apply.   White simply doesn't want to play a King's Indian defense.</p>

<p>With Nc3 White has a lot of options:  </p>

<ul>
<li>For one he can seek a more Dragon type game with the idea of Bc1-e3, 
Qd2, and h2-h4; often with f2-f3 being played.</li>
<li>Alternatively, White    can go for f2-f4 lines probably pushing
e4-e5.</li>
<li>Or White can play    classically with Nf3 and Be2.</li>
</ul>

<p>So overall a flexible developing move.</p>
 9321: <p> 3. c4 will likely transpose to a King Indian defense while 3. Nc3 will likely lead to a Pirc. 1. e4 players will prefer the more familiar Pirc positions.</p>

<p>The Modern Defense move order lets White decide the main position.</p>
