Why is the Three of Swords about pain?

Because the card's image says so, and the name behind the image is older than Rider–Waite.

The picture. Three swords driven through a heart, rain behind, no consolation in the frame — the most literal image in the deck. It isn't a metaphor layered on later; the earliest surviving printed tarot (the Milanese *tarocchi*, c. 1470s, Sforza court) has this card and calls it roughly *Laceration* — being torn/pierced. RWS in 1909 simply redrew what was already there.

The suit logic. Swords are air: mind, speech, judgment. So a wound *by three swords* reads as pain made by thought or by words — the heart hurt by what was said, or by what you couldn't stop concluding. Two Swords is the stalemate of not looking; Three is the moment the looking produces a cut. Three is the first number where a clean line crosses and something is actually pierced.

What it does and doesn't mean. Heartbreak, grief, separation, a wound still open — usually an event, not a slow condition. Reversed: the same pain re-opened and rehearsed rather than fresh. It is not a prediction that pain is coming, and never a statement about death.

Where it sits in the sequence. Ace cuts through confusion; Two stalls; Three bleeds; Four is the bed you rest in afterward — which is why Four of Swords is the practical follow-up to a Three, not another round of pain.

If you'd like, I can pull one card to show it in a position today, or explain how the Marseille deck renders the same card differently (some versions drop the heart).