3. C.
$x$ can take $1, 2, 3$, $y$ can take $2, 3, 4$, resulting in 6 complex numbers: $1+2i, 1+3i, 1+4i, 2+3i, 2+4i$, and $3+4i$, representing 6 points in the complex plane. Connecting each pair of points with a line, and counting only once those line segments that can coincide by translation, a total of 9 line segments are obtained, corresponding to 18 vectors. Therefore, 18 different differences can be obtained.
3. From the subset of complex numbers $\left\{x+y i \mid 1 \leqslant x<y \leqslant 4, x, y \in \mathbf{N}_{+}\right\}$, two different elements are chosen to perform subtraction. The number of different differences that can be obtained is ( ).
A. 30
B. 24
C. 18
D. 9