Premium Boxes! On level 5 !

Do you remember when they removed the 400 rp and they gave you in exchange these 2 premium boxes that you can get a skin on a champ that you want? If you firstly buy the champion and then open the premium box! Well i am here to ask in one of the posts of a Rioter he explained all the new levels and rewards that you get when leveling up in the new system! Well its written that you can the premium box on level 9 instead of level 5 as before but i got one premium box on level 5 before 5 mins on my new account! So you get Ashe , Garen and Brand i think on level 1,2,3 . So if i buy Lee Sin for example and i want a skin on him and open the premium box will these 3 champs have chance to get a skin onto? I want to main a certain champ but with the new system you can 3 champs before you get the box , so there is a chance that you get a skin on them from the box or do i get a skin on a champ that i bought after i got the box but didnt open? Can someone explain.