Step 1: Understanding Stoicism. 
  
Stoicism is a school of philosophy founded in Athens by Zeno of Citium. It emphasizes rationality, self-control, and virtue. Key figures in Stoicism include:  
- A. Seneca: A Stoic philosopher, known for his writings on ethics.  
- B. Marcus Aurelius: A Roman Emperor and Stoic philosopher, who wrote Meditations on Stoic philosophy.  
- C. Epictetus: A former slave who became a famous Stoic philosopher.  
- D. Protagoras: A pre-Socratic philosopher, but not associated with Stoicism; he was more aligned with relativism.  
Step 2: Conclusion.  
The correct answer is 3. A, B, C only.  
  
 
Final Answer:  
\[  
\boxed{\text{The correct answer is 3. A, B, C only.}}  
\]