3. Four Olympic medalists came to Xiao Wang's school to meet the teachers and students, two of whom were once Xiao Wang's classmates (the other two were not classmates). When the four medalists walked onto the stage one by one, the probability that the second person is Xiao Wang's classmate is ( ).
(A) $\frac{1}{6}$
(B) $\frac{1}{4}$
(C) $\frac{1}{3}$
(D) $\frac{1}{2}$
3. D.

Solution 1: Use "○" to represent classmate Xiao Wang, then all possible outcomes of the 4 people walking on stage in sequence can be enumerated using the tree diagram shown in Figure 3. There are 6 possible outcomes in total, and 3 of these outcomes have "○" as the second person, so the probability that "the second person is classmate Xiao Wang" is $\frac{3}{6}=\frac{1}{2}$.
Figure 3
Solution 2: Consider only the second person to walk on stage.
Any of the 4 participants could be the second one on stage, resulting in 4 possible outcomes, 2 of which are classmate Xiao Wang, so the probability that "the second person is classmate Xiao Wang" is $\frac{2}{4}=\frac{1}{2}$.