just curious, does it have to be 3? what if someone has 2 characters in the table and one of them is benjen and the player stands at 13 power, can he choose 1 character and kill the other one?

as I understand 'up to' it is far from 'exactly'

Correct, you can keep anywhere from 0-3. In one of my games my opponent intentionally (and thematically) burned Shireen in the same fashion just to get a kneel.

zomg THPOILERTHZ

that's what I thought as it happened in one of my games which left my opponent quite discombobulated, I think people just got used to wildfire killing anything above 3