3. (6 points) Dad is 24 years older than his son. This year, Dad's age is five times that of his son. $\qquad$ years later, Dad's age will be

three times that of his son.
【Answer】Solution: According to the problem, using the difference multiple formula, we get:
This year, when the father's age is five times the son's age, the son's age is: $24 \div(5-1)=6$ (years);
When the father's age is three times the son's age, the son's age is: $24 \div(3-1)=12$ (years); $12-6=6$ (years).

Answer: In 6 years, the father's age will be three times the son's age.
Therefore, the answer is: 6.