The Three of Swords is one of the few cards in the deck whose meaning almost nobody disputes, and its value lies in that unanimity.
The reading is grief, and specifically grief that has a source. The card does not describe depression, malaise or a general lowness of spirits; it describes a particular injury with a particular cause, a separation, a betrayal, a thing said, a piece of information that arrived and could not be put back. The three blades are usually taken as three parties or three causes rather than as a single wound, which is why the card so often turns up in questions involving a third person.
The distinction between this card and the more diffuse suffering cards is worth holding. A reader who collapses them loses the most useful thing the deck offers here, which is the ability to say that the pain is attached to something real and identifiable.
Its second and less obvious reading is clarification that hurt: the truth told, the diagnosis given, the illusion ended. On this account the card is not simply misfortune but the price of knowing, which fits the suit's association with the intellect better than the heartbreak reading does.
Waite gives dispersion, diversion and confusion for the reversed card, which is closer to grief becoming unfocused than to grief being cancelled. Readers who work without reversals reach the same distinction by asking whether the querent can name what happened to them. If they cannot name it, the card is usually doing something else, and forcing the grief reading onto a spread that does not support it is one of the commoner ways to lose a querent's confidence.