Cover Rules
"Big Creatures and Cover
Any creature with a space larger than 5 feet (1 square) determines cover against melee attacks slightly differently than smaller creatures do. Such a creature can choose any square that it occupies to determine if an opponent has cover against its melee attacks. Similarly, when making a melee attack against such a creature, you can pick any of the squares it occupies to determine if it has cover against you."
So, in your examples, as long as both combatants stay in line with the door, none of them have cover, since they can always just chose the square that is in a direct line through the door from the opponent. (Assuming the door is at least 5 feet wide. If it is narrower, there will always be cover)
The only tricky part is wether the melee part in the above statment holds true for reach as well: "When making a melee attack against a target that isn't adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks." - If that statement prevails and you are using the rules for ranged attacks, in 2 and 3 there will be cover, but only in favor of the large creature since the medium one cannot draw uninterrupted lines to all corners of the large one, but the large one can chose a close corner and draw uninterrupted lines to all corners of the medium creature.
