Exter, magnus, diduus, they all belong to the first declension and also the second declension.
Why these adjectives are so special?
This is, perhaps surprisingly, how the vast majority of Latin adjectives work!
In general, there are two types of Latin adjectives: "first-second" and "third". These are all "first-second" adjectives, which take first-declension endings in the feminine, and second-declension endings everywhere else.
Note that this is based on the noun's gender, not its own declension: qvercus magna (f), nauta magnus (m), vulgus magnum (n).