An observation that describes why we should be sure we understand what questions and challenges Plato, Aristotle, and others had raised about pleasure prior to Epicurus: "... [T]he besetting vice of philosophy is to abstract propositions away from the context of the practical problems and questions that gave rise to them. Until we know the practical context of problems and questions to which a proposition is supposed to be an answer, we do not know what it means. ... Applying this doctrine to the interpretation of historical texts, Collingwood insists that you cannot know what a philosopher meant by a doctrine until you know the question to which the doctrine was intended as an answer and how that question arose."
Example: Is there a relationship here? If not, what other questions might provide the context for PD3?
SOCRATES: Have pleasure and pain a limit, or do they belong to the class which admits of more and less?
PHILEBUS: They belong to the class which admits of more, Socrates; for pleasure would not be perfectly good if she were not infinite in quantity and degree.
SOCRATES: Nor would pain, Philebus, be perfectly evil. And therefore the infinite cannot be that element which imparts to pleasure some degree of good. But now — admitting, if you like, that pleasure is of the nature of the infinite — in which of the aforesaid classes, O Protarchus and Philebus, can we without irreverence place wisdom and knowledge and mind? And let us be careful, for I think that the danger will be very serious if we err on this point.
Epicurus: PD 3. "The limit of quantity in pleasures is the removal of all that is painful. Wherever pleasure is present, as long as it is there, there is neither pain of body nor of mind, nor of both at once."
As to the historical setting at the time of Epicurus, this was offered on Facebook as a reference work, and looks pretty good: