¡Three Amigos! - Notice how we keep seeing the same three Amigos running around during the climax, but when the villagers all appear, their uniforms look far less detailed and, well, they're not played by the three leads? That's because to the confused banditos, they all looked like the real amigos, and we see what they're seeing during the gunfight.
Where did the villagers get all those guns to fight off the banditos at the end, and if they always had that many weapons handy, why did nobody in Santo Poco ever try defending themselves before?
They have no gun skills and they're not murderers. Notice they're not aiming. They're just trying to confuse the bandits.
Self-defense generally isn't the same as murder, but the question works in reverse, too: If they're too pacifistic even for self-defense, why'd they have so many revolvers lying around in the first place? Ordinary handguns aren't exactly ideal hunting pieces even if the wielder is skilled in their use.
Probably family heirlooms from a time when the ancestors were not as peaceful as the living descendants.
Before the climactic showdown, Carmen's father says "We want to defend ourselves, but how?" It's most likely that El Guapo and his gang had them cowed through fear, and they didn't fight back despite having weapons because they were afraid of what would happen if they failed. Since we only see Lucky, Dusty and Ned running around during the climax (why is noted above), we don't know for certain who was shooting down most of the banditos but are left to assume that the villagers racked up a few kills on their own, just because the three Amigos themselves couldn't have the only ones attacking from the sheer number of positions we see the banditos taking fire from.