3. In the expansion of $(x+y+z)^{8}$, the sum of the coefficients of all terms of the form $x^{2} y^{a} z^{b}(a, b \in \mathbf{N})$ is
A. 112
B. 448
C. 1792
D. 14336
In the product of 8 parentheses, except for selecting $x$ from 2 parentheses, the rest of the parentheses can select one from $y, z$, the number of ways is $C_{8}^{2} \cdot 2^{6}=28 \cdot 64=1792$, so the answer is $C$.