The plot card holding the trident seems overpowered. If I am playing a three person game and the person who plays this card has less characters than only one of them, do they still get +2 strength

Yes, you do not have to have the fewest number of characters of any player to get the bonus. You just cannot have the greatest number of characters.

I'm of the opinion that that wasn't the original intention, but since this card they have used the wording "…than each opponent" whenever they want you to be the one with the least/most of something, whatever the case may be. As worded, you can have 9 characters, as long as someone else has 10 or more… (probably a bit of an exageration, but you get the idea)