I want to learn a black first move which is compatible against e4, d4 and c4 and is not too theoritical. I have been thinking about g6 and d6. Are they any good?

i recommend 1...nf6  this is a flexible first move that works against everything.  if white plays 1. c4 or 1.d4 you will get some kind of queens pawn game and against 1. e4 you will usually get an alekhine opening.  i have found that i have been able to get to familiar comfortable positions out of the opening as black using this strategy

Isn't nf6 theortical? Right now I'm playing the nimzo/queen's indian and it requires some study. Nf6 against d4, c4 and e4 have different types of positions. I'm looking for a one-size-fits-all move which utilise similar strategies against all white first moves.

if you're looking of such system i would suggest 1... b6 (owen's defense/ english defense) on everything. This is not particular good and some will even call it bad but it's playable and fits your condition of a one size fits all move.

+: there's very little theory and your opponents are probably not aware that this is a move. Also you can play it as white with 1 b3 the larsen opening.

-: with normal play black has a serious space disadvantage so be aware of this.

what do you mean ignore ...Nf6 because it is highly theoretical?  There are only so many choices white can make.  That means you can force white into about 6 different variations that you know and are comfortable with, for example I force my opponents into variations of Alekhine's defense or the Queen's Gambit Declined, Classical Variation.  The pirc is worse than alekhine's defense.  Of course you are not playing the same opening each time, even if you play ...d6 you can't play the same system every time.  

1...b6 is really not as bad as people make it out to be. You just need a good plan on what to do and I think most people play it like idiots thinking they will just feel their way through it with no prep and no study. Like any opening at not GM level if you just know what you are trying to do it is more then effective enough to play. 

The Pirc is totally a joke though. People just need to learn to laugh and not take it so seriously all the time.

saying the pirc is worse than alekhine's defence is not absurd lol.  alekhine's defence isn't that hard to play, basically only the four pawns attack can get tricky as black and plus you benefit from most white players not being very familiar with the alekhine.  i have had many games with the alekhine defence as black where i fork white's bishop and knight with a pawn on e4 because white played a natural looking knight development at the wrong time.

pirc is so easy to play against with white.  just set up a perfect pawn center with e4 and d4, develop your knights, maybe put your bishops on e3 and d3.  There is no such thing as a universal 'system'  All openings rely on some theory, but with ...Nf6 you can force your opponent into lines that you are comfortable with and you have better attacking chances than if you play the ugly looking ...d6

1...e6

i thought about suggesting that move as well, it is a nice flexible opening can transpose into a French defence or a Queen's gambit Declined classical variation.  I actually used to play 1...e6 against every white first move, but I switched to 1...Nf6 because I found I prefer the Alekhine defence to the French defence.

Well if most people think that way, then it must be true right?  Here, this might help you out next time you are trying to make a point: http://en.wikipedia.org/wiki/Argumentum_ad_populum

Anyways, Nf6 is a strong first move for black against e4, it doesn't make life 'really hard.'  You give no examples about Nf6 making life 'really hard' anyways.  You do make a good decision to decide to not argue anymore, since you can't make any points of your own or address any of my points.  Better luck next time 

1..e6.  I go for 1..e6 especially against 1.d4.  With 1.e4 e6, one may counter e6 with 2.Nf3 which may not be Black's idea and may get into the French defense/wing gambit or else the exchange variation.

Universal system for black that is not too theoretical

how about this one? 1...Nc6. I haven't read the book but they say its good

why not learn proper mainlines?

Im sure the amount of study will be the same .

It's not like 1 move will narrow down everything because it will branch out anyway.

If you are not ready for some theoretical knowledge,Im sure you can find alot of simple systems within  those mainlines

Let's make it happen!

You could try 1...c6 against any of the above, with the idea of following up with 2...d5.  This would entail learning the Caro Kann and/or the Slav which are both nice tries, but "theoretical"

Everything is eventually theoretical. There is no quick fix to chess.

It's probably better to learn a few opening principles to follow, rather than a default move or moves.  This will get you farther as you advance in skill.

If you choose to play a "system" rather than learning openings, I would suggest learning at least two, so that you have one for d4 openings and one for c4 openings.  

Youtuber "GJ Chess" has a video called "Universal Chess Opening against d4."

Against e4, you could look at a video called "Chess Opening for Black: A System V Systems!" by Melbourne Chess Club, but use with caution because it resembles a sicilian defense and a lot of players are very familiar with the sicilian, so I'm not sure how wise it would be to rely on a pre-planned system.  A better alternative against e4 might be to learn to play the French Defense, a strong defense that does not have as many complex lines as say the Sicilian.

The Modern Defense works against everything except for 1.b4 or 1.b3. I play the Pirc, KID, and Modern and since they're all interconnected it works great and I can play well against any white first move.

I know this an old discussion but the 1...d6 is much more system-like than 1...nf6

Scandinavian and Slav is your best bet. You will use the c6,d5, sometimes e6 pawn moves. This will cover 1.e4, 1.d4, and 1.c4

Why the Scandinavian instead of the Caro Kann? Simple. The Scandinavian is less work. You can graduate to the Caro Kann.

Which Scandinavian? The trickiest one to meet that is GM tested and increasing in popularity is the Gubinsky-Melts Variation. Get the inside story and join us at http://www.chess.com/groups/view/scandinavian-defense-3qd6-variation