How to use reworked irelia's Q????

hi.So see,the thing is her q was shown going through 3 minions in one time.However im unable to do this on champions.It says they need to be marked for it.but then why is it working on minions? Is the only way to use her q like that is to use her E or R to mark the champs or is there another way to use it the same way on champs like she can on minions?? link to her using it on minions:https://www.youtube.com/watch?v=h8iqfTg75Wk