Explanation:
In the sentence "My friend, Sarah, won the competition," there are three nouns:
1. **Friend**: This is a **common noun**. It refers to a general person who is a companion or acquaintance.
2. **Sarah**: This is a **proper noun**. It refers to a specific person and is always capitalized.
3. **Competition**: This is a **common noun**. It refers to a general event where people compete against each other.
