3. A. If $x>1, y>0$, and satisfy
$$
x y=x^{y}, \frac{x}{y}=x^{3 y} \text {, }
$$

then the value of $x+y$ is ( ).
(A) 1
(B) 2
(C) $\frac{9}{2}$
(D) $\frac{11}{2}$
3. A. C.

From the given equations, multiplying them yields
$$
x^{2}=x^{4 y} \Rightarrow y=\frac{1}{2} \text {. }
$$

Furthermore, $x=4$.
Therefore, $x+y=\frac{9}{2}$.