It's only been 3 lessons but this happens times to times. Sometimes, students really don't have time to practice. The parents are not involved or they had extra school work, or out of town visitors, the list goes on. I let them know right up front that I know they haven't practiced, they need to know that you know they didn't practice . I am very gentle and ask, "You are having trouble playing today, did you get a chance to practice?" Of course they say no and then I ask why. Not only does it give me an insight into how their parents are involved, it also lets me know more about the student. It also lets them what I expect out of them. We then practice what they should have practiced at home, I sit with them and watch them play so I can see if they are reading the notes, if maybe they didn't understand something the week before. I go over HOW to practice. AND I do not give them a sticker for that week. I am always surprised how important stickers are to students up to the age of about 10.
I had one boy that consistently did not practice. He would start playing and then I would say, "You didn't practice much this week?" He would give me a reluctant, "No." He was older, this was becoming a habit and I needed something more extreme. Out loud I said, "Hmmm, I figured since you didn't practice at home, you will practice here, I want you to play the song 10 times in a row. " He was shocked but began playing. As he played, I would clean up the studio. The trick is to not sit there and watch them practice, but listen to them practice and let them know you are listening while doing something else. Every now and then I would said, "7 more times to go." Before he left, I said to him, "Remember practice at home, or we spend the lesson practicing here." The following week, he proudly announced he had practiced.
Also, don't forget sometimes not practicing can be a sign that the student is getting frustrated or bored. You got to make sure they know their notes or they won't play. Also, a great way to get them going is to let them play Old MacDonald or Twinkle Twinkle. I guarantee they will come back the following week, knowing that song.
P.S. While your doing the flash cards, do them once visually, then the next time, have the student look at the flash card and play the note. Otherwise they will memorize the card but not where the note is located on the piano.