1 Nc3 is a solid move, develops a piece, controls centrral squares e4 and d5. Any white move is playable. You cannot refute it.
Transitions are fine: 1...c5 to Sicilian, 1...e5 to Vienna, 1...Nf6 to Veresov, 1...d6 to Pirc-Ufimtsev, 1...d5 to Scandinavian, 1...e6 to French, 1...c6 to Caro-Kann.
Sharpest is 1...d5, but after 2 e4 black should not push 2...d4.
Compare with 1 d4 Nf6 2 c4 g6 3 Nc3 Bg7 4 e4 d6 5 Nf3 O-O 6 Be2 e5 7 O-O Nc6 8 d5 Ne7 9 Nd1 Nd7 10 Nd3 f5 11 f3 Nf6. It is the same manoeuvre, but with 3 tempi advantage for the 1 Nc3 player: 1 from playing white instead of black and 2 because the Ng8-f6-d7-f6 happens in one move Ng1-f3 after white plays f4.
After 1...d5 2 e4 dxe4 3 Nxe4 looks better. Black can the go to Caro-Kann with 3...c6, French with 3...e6, but he can also play 3...e5. Look at this: after 1 e4 d5 2 exd5 Qxd5 3 Nc3 Qd8 white will never play 4 Ne4.
Transitions are fine: 1...c5 to Sicilian, 1...e5 to Vienna, 1...Nf6 to Veresov, 1...d6 to Pirc-Ufimtsev, 1...d5 to Scandinavian, 1...e6 to French, 1...c6 to Caro-Kann.
Sharpest is 1...d5, but after 2 e4 black should not push 2...d4.
Compare with 1 d4 Nf6 2 c4 g6 3 Nc3 Bg7 4 e4 d6 5 Nf3 O-O 6 Be2 e5 7 O-O Nc6 8 d5 Ne7 9 Nd1 Nd7 10 Nd3 f5 11 f3 Nf6. It is the same manoeuvre, but with 3 tempi advantage for the 1 Nc3 player: 1 from playing white instead of black and 2 because the Ng8-f6-d7-f6 happens in one move Ng1-f3 after white plays f4.
After 1...d5 2 e4 dxe4 3 Nxe4 looks better. Black can the go to Caro-Kann with 3...c6, French with 3...e6, but he can also play 3...e5. Look at this: after 1 e4 d5 2 exd5 Qxd5 3 Nc3 Qd8 white will never play 4 Ne4.