Well, if a person is equally willing to go to point A and point B, but not to point C, why can't they indicate this in the vote?
Added 1 minute ago
In fact, those who vote for 2 cities are essentially voting against the third. And if they vote for 3, it's the same as not voting at all.
Well, if the option for multiple choices is really needed, then Merlin's option could work, i.e., a person can distribute the sum of their points between two cities, instead of giving a full vote to each, which would give the person two votes.
And, as another option, someone already mentioned it, not to count the votes of local residents.