Defence skill: The unit's skill in defence, if it had no armour or shield.
Armour: The armour for that unit, including upgrades, which is added on to the defence skill.
Shield: The shield for that unit which is added on to the defence skill.
They're all added together. All the units you detailed about would have twenty as their overall defence.
To attack the first, use arrows as it has no shield and armour. For the second, use a powerful melee charge as it has no shield (could also use missiles). For the third, use missiles again, but if something had no defence skill then there are really no melee units that it could stand up to. Of course, the situation won't happen.
Defence, armour, and shield are added together, and cannot exist independently.
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