TLDR: you have to get the costume yourself, it wont be given to you (unless this has been changed which i doubt)
1st costume = from adventurers box (you have that it seems)
2nd costume = from adventurers box at level 10 (?) (you have that it seems)
3rd costume = from costumebox (you can get from anniversary-NPC for anniversary coins, from itemshop, from costumesale, from auction, from daily mission box,...) (you want that it seems)
(when you create a char you can have a preview how it looks at later levels; there are 3 ranks, thats where i take the numbers from)
in general, there is a costume which has a rank between the 2nd and the 3rd, its from prophetic cards
(some NPCs which represent their class wear that costume, for example the NPC warriors (guardians) at nezebgrads port)
however i noticed for xadaganian engineer the "normal" 3rd costume is only available from prophetic cards (or global sale) while there is no specific set for the rank between 2 and 3