Rio de Janeiro
Off the top of my head -- when you use somebody's location, you place one of your faction icons on their card to mark that you've already used it. Then you hand them the worker, which they can then use for something else themselves on that turn.
You could conceivably send one worker to each one of your three opponents, which is why you can do it up to three times per turn.
You can also send your workers to three different locations of the same opponent.