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
3. C.

Notice that, $(x+y+z)^{8}=\sum_{k=0}^{8} \mathrm{C}_{8}^{k} x^{8-k}(y+z)^{k}$. Then the sum of the coefficients of all terms of the form $x^{2} y^{a} z^{b}(a, b \in \mathbf{N})$ is $\mathrm{C}_{8}^{6} \times 2^{6}=1792$.