I love these puzzles. GNU C supports a label as value for computed goto. This is useful for direct threaded dispatch. You trade off a branch instruction for an address lookup, but it makes the code more structured.

  int main(void) {
    void* A[] = {&&A0, &&A1, &&A2, &&A3};
    void* B[] = {&&B0, &&B1, &&B2, &&B3};
    void* C[] = {&&C0, &&C1, &&C2, &&C3};
    goto *A[SCAN];
    A0: WRITE(1); RIGHT; goto *B[SCAN];
    A1: WRITE(3); LEFT ; goto *B[SCAN];
    A2: WRITE(1); RIGHT; HALT; return 0;
    A3: WRITE(2); RIGHT; goto *A[SCAN];
    B0: WRITE(2); LEFT ; goto *C[SCAN];
    B1: WRITE(3); RIGHT; goto *B[SCAN];
    B2: WRITE(1); LEFT ; goto *C[SCAN];
    B3: WRITE(2); RIGHT; goto *A[SCAN];
    C0: WRITE(3); RIGHT; goto *B[SCAN];
    C1: WRITE(1); LEFT ; goto *B[SCAN];
    C2: WRITE(3); LEFT ; goto *C[SCAN];
    C3: WRITE(2); RIGHT; goto *C[SCAN];
  }