1. **Problem:** You are asking why, in a union graph, we only show $3$ in the middle. 2. **Formula used:** The union is written as $A\cup B$ and means all elements in $A$, in $B$, or in both. 3. **Important rule:** If an element appears in both sets, we write it only once in the union. 4. **Why $3$ is in the middle:** In a Venn diagram, the middle part represents the overlap, so elements that are in both sets go there. 5. **Work:** If $3$ is in both sets, then it belongs in the overlapping region. $$A\cap B=\{3\}$$ $$A\cup B=\{1,2,3,4,5\}$$ 6. **Simple explanation:** We are not saying $3$ is the union. We are saying $3$ is one element that belongs to both sets, so it is drawn in the middle. 7. **Final answer:** We show $3$ in the middle because it is common to both sets, and the middle part is where overlap goes.