3. A certain section of a road consists of three equal-length segments: an uphill, a flat, and a downhill section. It is known that a car travels at average speeds of $v_{1}, v_{2}, v_{3}$ on these three segments, respectively. Then, the average speed of the car over this section of the road is ( ).
(A) $\frac{v_{1}+v_{2}+v_{3}}{3}$
B) $\frac{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}}{3}$
(C) $\frac{1}{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}}$
(D) $\frac{3}{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}}$
3. D.

Let the length of this section of the road be $3 s$. Then the time taken by the car to travel on each section is $t_{i}=\frac{s}{v_{i}}(i=1,2,3)$. Therefore, the average speed of the car on this section of the road is
$$
\bar{v}=\frac{3 s}{t_{1}+t_{2}+t_{3}}=\frac{3 s}{\frac{s}{v_{1}}+\frac{s}{v_{2}}+\frac{s}{v_{3}}}=\frac{3}{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}} .
$$