If you get them at a feed store, most likely not sexed.
Ducks do best with a friend, so if you do get them, get 2.
Keeping and raising together can be done, but watch carefully... the chicks can peck the ducklings pretty badly, especially if they are already 3 weeks.
You may have to provide separate housing, but may be able to house them together. It really depends and you won't know for sure until you try it.