Yes, $3 \in \{1, 2, 3\}$.

The symbol $\in$ represents "is an element of." Since the set $\{1, 2, 3\}$ contains the number 3, it is correct to say that 3 is an element of that set.

Do you need more details on sets or the concept of membership?

Here are some related questions:

1. What is the difference between a set and a subset?
2. How do you express set membership using symbols?
3. What does the symbol $\notin$ mean in set theory?
4. Can a set contain other sets as elements?
5. How do you describe the union or intersection of two sets?

Tip: A set can be defined explicitly by listing its elements, or it can be defined by a rule or condition (e.g., $\{x \mid x \text{ is a positive integer}\}$).