3. Person A and Person B start from the same point $A$ on a circular track at the same time and run in opposite directions. Person A's speed is $5 \mathrm{~m}$ per second, and Person B's speed is $7 \mathrm{~m}$ per second. They stop running when they meet again at point $A$ for the first time. During this period, they meet a total of $n$ times, so $n=$ $\qquad$
3. 12 .

Two $\lambda \mu A$ start from $A$ and meet once (not at point $A$). At the moment of their first meeting, the two have run a certain distance. From the ratio of their speeds, it can be deduced that person A has run $\frac{5}{12}$ of a lap, and person B has run $\frac{7}{12}$ of a lap. Therefore, after the $n$-th meeting, person A has run $\frac{5}{12} n$ laps, and person B has run $\frac{7}{12} n$ laps. When they meet again at point $A$, both person A and person B have run an integer number of laps, so $\frac{5}{12} n$ and $\frac{7}{12} n$ are both integers. The smallest value of $n$ is 12, thus they have met 12 times.