3 C. First, fill in the number in the 1st square, there are 3 ways that meet the conditions. When the 1st square is filled with $i(2 \leqslant$ $i \leqslant 4$ ), then fill in the $i$-th square next, there are also 3 ways that meet the conditions. The third step is to fill the remaining two numbers into the remaining two squares, and there is only 1 way that meets the conditions. By the multiplication principle, there are $3 \times 3 \times$ $1=9$ ways that meet the conditions.
3 Fill the numbers $1,2,3,4$ into four squares labeled $1,2,3,4$, with one number in each square, such that the label of each square and the number filled in it are different. The number of such filling methods is
A. 6
B. 8
C. 9
D. 16