3. The side length of rhombus $A B C D$ is 1, and its area is $\frac{7}{9}$. Then the value of $A C+B D$ is ( ).
(A) $\frac{4}{3}$
(B) $\frac{16}{9}$
(C) $\frac{8}{3}$
(D) $\frac{32}{9}$
3. C.

Let $A C=2 a, B D=2 b$. Then
$$
\begin{array}{l}
a^{2}+b^{2}=1,2 a b=\frac{7}{9} \Rightarrow(a+b)^{2}=\frac{16}{9} \\
\Rightarrow a+b=\frac{4}{3} \Rightarrow A C+B D=\frac{8}{3} .
\end{array}
$$