Step 1: Understanding Stoicism.
Stoicism, founded in Athens by Zeno of Citium, is a philosophy stressing rationality, self-control, and virtue. Notable Stoics include: - A. Seneca: Esteemed for his ethical writings. - B. Marcus Aurelius: Roman Emperor and philosopher, author of *Meditations*. - C. Epictetus: A former slave who achieved renown as a Stoic philosopher. - D. Protagoras: A pre-Socratic philosopher associated with relativism, not Stoicism.
Step 2: Conclusion. The correct choice is 3. A, B, C only.
Final Answer: \[ \boxed{\text{The correct answer is 3. A, B, C only.}} \]