The line between what's yours and what isn't
All three build their thinking around the same core distinction: judgment, will, and reaction belong to you; outcomes, reputation, and other people's actions do not. Epictetus states it as bare doctrine, Marcus repeats it to himself in a war camp, and Seneca dramatizes it in letters to a friend — but it is the same load-bearing wall in every case.
Death rehearsed, not feared
Each treats mortality as something to be prepared for in advance rather than avoided in thought. Marcus wrote nightly reflections on it from the edge of an empire in crisis, Epictetus taught it as a discipline to strip away false attachment, and Seneca staged his own death as a final, literal demonstration of the argument.
Philosophy as practice, not theory
None of the three produced a detached system for its own sake. Marcus's Meditations were private exercises, Epictetus's teachings were spoken lessons for people navigating real constraint, and Seneca's Letters were addressed to an actual friend trying to live better. The table would agree that philosophy that doesn't change conduct isn't philosophy at all.