11. This year, Xiao Jun is 5 years old, and his father is 31 years old. In $\qquad$ years, his father's age will be 3 times Xiao Jun's age.
【Analysis】According to "this year, Xiao Jun is 5 years old, and his father is 31 years old," the age difference between father and son is (31-5) years. Since this age difference does not change, the multiple difference is 3-1=2. Therefore, using the difference multiple formula, we can find Xiao Jun's age when his father's age is three times his. From this, we can further solve the problem.

【Solution】Solution: The age difference between father and son is: $31-5=26$ (years),
When the father's age is three times Xiao Jun's age,
Xiao Jun's age is: $26 \div(3-1)$
$$
\begin{array}{l}
=26 \div 2 \\
=13 \text { (years), }
\end{array}
$$
$$
13-5=8 \text { (years), }
$$

Answer: In 8 years, the father's age will be three times Xiao Jun's age.

Therefore, the answer is: 8.
【Comment】The key to solving this problem is based on the fact that the age difference between the two people does not change over time. Using the difference multiple formula to find the corresponding age of the son, we can solve the problem. The relationship formula for the difference multiple problem: Quantity difference $\div$ (multiple -1$)=1$ multiple (smaller number), 1 multiple (smaller number) $\times$ multiple $=$ several multiples (larger number).