How does one play against 2. d4 cxd4 3. Nf3 in Sicilian? I'm wondering about the move order 2. d4 cxd4 3. Nf3 in Sicilian. If black does not try to protect the pawn and plays some normal move like 3... d6, 3... e6 or 3... Nc6, then 4. Nxd4 transposes into one of the usual lines. On the other hand,  3... e5 does not seem to be the option, as e. g. white can play 4. c3, and if black captures on c3, it's a Smith-Morra gambit which black has already messed up by playing d5 in one move. 

So, are there any options for black that are better than transposing to the main lines? If not, I'm wondering why this move order is much less popular than 2. Nf3.
It seems you only give Black extra options compared to the usual 2.Nf3 and 3.d4


Black can reach whichever Open Sicilian he was aiming for. He has made no concession since ...cxd4 was part of the plan anyway.
As pointed by BlindKungFuMaster, 3...Nf6 will lead either to an Alapin, or to a harmless sub line of the Nimzo-Sicilian (1.e4 c5 2.Nf3 Nf6 3.e5 Nd5, and now 4.Nc3 is considered much better than 4.d4). The small plus side for you is if you want to play an Alapin yet avoid lines without ...cxd4, e.g. if your opponent likes to play 1.e4 c5 2.c3 e6 3.Nf3 Nc6 4.d4 d5: this interesting option is not available anymore for Black.
3...e5!? is actually perfectly playable. After 4.c3 Black doesn't have to take the pawn but can continue with 4...Nc6. This is actually one of Sveshnikov's recommendations for Black vs the Morra in his recently published repertoire against the anti-sicilians.