3. (5 points) This year, Dandan is 4 years old, and Dandan's father is 28 years old. In $a$ years, the father's age will be 3 times Dandan's age. Then the value of $a$ is . $\qquad$
【Analysis】According to "Dandan is 4 years old this year, and Dandan's father is 28 years old", we know that the age difference between Dandan's father and Dandan this year is $28-4=24$ years. Since the age difference does not change with time, using the difference multiple formula, we can find Dandan's age when her father's age is 3 times Dandan's age by dividing 24 by the difference in multiples (3-1), and then find the answer.

【Solution】Solution: Age difference: $28-4=24$ (years),
Dandan's age: $24 \div(3-1)$
$$
\begin{array}{l}
=24 \div 2 \\
=12 \text { (years), } \\
12-4=8 \text { (years), }
\end{array}
$$

Therefore, the value of $a$ is 8.
Answer: In $a$ years, Dandan's father's age will be 3 times Dandan's age, so the value of $a$ is 8.
The answer is: 8.