1. **Problem:** You are asking whether $3$ is the union, or whether union means both joining. 2. **Formula used:** The union of two sets is written as $A\cup B$. 3. **Important rule:** Union means “all elements that are in $A$, in $B$, or in both.” 4. **Check the example:** If $A=\{1,2,3\}$ and $B=\{3,4,5\}$, then $3$ is not the union by itself. 5. **Work:** $$A\cup B=\{1,2,3,4,5\}$$ Here, $3$ is only one element that belongs to both sets. 6. **Meaning in simple words:** Yes, union means both sets joined together, and if something is in both sets, you write it only once. 7. **Final answer:** No, $3$ is not the union. Union means both sets combined, so $A\cup B$ includes everything from both sets.