3. C Among them, selecting one child to take the red ball has 3 ways, the probability of taking the red ball is $\frac{3}{10}$, and the probability of not taking the red ball is $\frac{7}{10}$.
3. Among 10 balls, 3 are red. If the balls are randomly distributed to 10 children, each getting one ball, then the probability that exactly one of the last three children gets a red ball is
A. $C_{4}^{4} \cdot \frac{3}{10}$
B. $\frac{C_{3}^{1} C_{7}^{2}}{C_{10}^{3}}$
C. $C_{3}^{\mathrm{t}} \cdot \frac{3}{10} \cdot\left(\frac{7}{10}\right)^{2}$
D. $\frac{3}{10} \cdot\left(\frac{7}{10}\right)^{2}$