3. If the focal distance, minor axis length, and major axis length of an ellipse form an arithmetic sequence, then its eccentricity is ( ).
(A) $\frac{3}{4}$
(B) $\frac{\sqrt{2}}{3}$
(C) $\frac{3}{5}$
(D) $\frac{9}{10}$
3.C. From $2 a+2 c=4 b, a^{2}=b^{2}+c^{2}$. Eliminating $b$ yields $3 a=5 c$.

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3.C. From $2 a+2 c=4 b, a^{2}=b^{2}+c^{2}$. Eliminating $b$ yields $3 a=5 c$.