3. It is known that the ages of A, B, and C

are all positive integers. A's age is twice B's age, B is 7 years younger than C. If the sum of the ages of these three people is a prime number less than 70, and the sum of the digits of this prime number is 13, then the ages of A, B, and C are
3. A is 30 years old, B is 15 years old, C is 22 years old.

Let the ages of A, B, and C be $x$ years, $y$ years, and $z$ years respectively. According to the problem, we have
$$
\left\{\begin{array}{l}
x=2 y, \\
y=z-7, \\
x+y+z \text { is a prime number, and } x+y+z<70 . \\
\because 13=4+9=5+8=6+7,
\end{array}\right.
$$
$\therefore$ The prime number less than 70 and whose digits sum to 13 is only 67,
i.e., $\square$ $x+y+z=67$.
In conclusion, solving the equations yields $x=30, y=15, z=22$.