Preferred 3rd move for white in Queen's gambit accepted

After 1.d4 d5 2. c4 dxc4, what should I move next? I used to prefer Nc3, but I came a trap when I play 3.e3 and he replies with b5?, and I can take threaten the rook with my queen and win a knight. But in the opening explorer, the most common move by people and masters are Nf3, this will block the f3 square for my queen if they fall for the trap. 

I think 3.e3 is the most reliable option. It can transpose to 3.Nf3 lines without giving black the annoying Bg4 option. e4 would have been great except it gives black too many breaks. Still a lot of people like it.

Thats my take anyway. :-)

But my Dark squared bishop gets trapped by the pawns

You could try 3.Nf3 4.g3 5.Bg2 6.0-0 and you have long term diagonal pressure. Follow with Ne5 if possible opening the bishop and attacking c6 twice, Qc2 to hit and possibly recapture the c4 pawn. If black plays c6, b5, a6, Bb7 trying to hold the pawn on c4 undermine his chain with a4 and b3. Try to make your whole series of moves about pressuring the diagonal and this will be much more powerful and difficult to face than a one move threat of Qf3 which black may or may not miss.

Temporarily, yes, but thats common in many QGD and slav lines, you shouldnt worry about it. The bishop will come out via b2 or a future e4. 

3.Nc3 has some tactical problems. 3...a6 is also good against it. 3.Nf3 Nf6 4.Nc3 is another story.

So, my best third move is Nf3? 

It depends on what kind positions you want to play. The only moves I think White is supposed to avoid are 3.Nc3 and 3.Qa4+.

3.e3 is the safest if you want to get the pawn back with no trouble. 3...Be6 is rare but it can be played so you should look at some games with that.

3.e4 leads to open positions similar to what you get after 1.e4 e5.

3.Nf3 lets you play 4.e3 next without allowing Black to play ...e5.

3.Nf3 Nf6 4.Nc3 usually leads to positions where White plays the position like a true gambit. White wants to play e4 and get a big center. Black usually plays 4...a6 or 4..c6 then to try and hold the pawn. 4...c6 5.a4 can lead to a Slav defense which is nice if you happen to play this line with White.

 Well, all of the mentioned 3rd moves are equal, since either you or your opponent will hang material later in the game almost certainly, making any of these little considerations useless.

This is true, but white can sac the knight on move 6 in this line. If black takes the knight he's in big trouble. If black refuses (and plays accurately) then he gains a slight edge but it's still playable for white.

This is good but after 4.e3 it's actually better for Black to play 4...Nf6 and transpose to the 3.Nf3 main line where the White knight has moved to c3 too early. Whenever Black decides to play ...c5 and ...b5 he'll have the extra possibility of ...b4 hitting the knight which he wouldn't have in the normal position.

I play 3. e3 simply because I love this kind of positions and I don't care that my black squared bishop will stay behind the pawn chain because I will play b3 or a3 and b4 and Bb2 where the bishop will be great piece (even more if black castle short, and it will combine well with the c4 bishop). I prefer e3 over Nf3 because Nf3 gives black more options. 

Hope I have helped you

4... Nf6 is a practical choice for black and it seems to avoid the complications of the 4... b5 line, but I don't think it's objectively better. At least 4... Nf6 develops a piece and results in a more normal looking position. The 4... b5 line is more aggressive and black had better know the theory to get into it. After 4... Nf6 white isn't getting any advantage out of the opening so it's a fine choice, I just take issue with calling it "better" than 4... b5.

And really white shouldn't be playing 3 Nc3 anyway. 3 e3 or 3 Nf3 are the solid responses for white, and 3 e4 is more ambitious. 3 Nc3 is potentially tricky though.

Nc3, Nf3, e3, e4, g3(yes), and even Qa4 are playable.  Probably in that order.  The first 4 choices are interchangeable and depend on which type of game you are playing for.  G3 followed by Bg2, Nf3, and 0-0 will lead to Catalan like positions.  Qa4 is not particularly ambitious and violates some principles but when played properly will give you a playable, and often unknown, middle game position.  It still should be your last option.

i play e3. it's impossible for black to keep the pawn without losing material.  https://www.youtube.com/watch?v=bgVzgG1mQTE

This is isn't White's best though. Usual is 4.Nf3 ed 5.Bxc4 Nc6 6.O-O.

He really likes 3.e4. I don't think it's an actual evaluation of the move.

1. d4 d5 2. c4 dxc4 3. Nf3 Nf6 4. e3 e6 5. Bxc4 c5 6. O-O a6  
1. d4 d5 2. c4 dxc4 3. e4 e5 4. Nf3 exd4 5. Bxc4 Bb4 6. Nbd2 
1. d4 d5 2. c4 dxc4 3. e4 e5 4. Nf3 exd4 5. Bxc4 Nc6 6. O-O

Generally speaking 3. Nf3 and 3. e3 transpose.
With 3. e4 black often varies on move 5. 

In the 3. e3 variation there is 3. ... b5 (?!) 4. a4 c6 5. a4xb5 c6xb5 6. Qf3 or 4. ... a6 5. a4xb5