When you see this abbreviation in your child's book or notebook, it means more than simply to practice the song three times. I have instructed your child to practice the song, or more likely a section (or sections) of the song until he/she is able to play that section three times in a row without error! Most students stop practicing when they get it right once. But getting it right once isn't enough; it might have been just good luck!
When I ask students to do this for me in lessons, it is amazing how often they are able to play a passage once or twice without error, but then they make a mistake on the third time. What do I tell them to do? Start over! Keep playing that passage until the get it right 3 times in a row! Additionally, I recommend they practice this way daily. They need to be able to get it right 3 times in a row on Monday, and on Tuesday, etc.
This is a great practice technique for the child who seems to practice their songs very quickly at home. You might suggest: Why don't you try to play that song 3 times without a mistake? Can you do that?
If your child is really struggling with something, they should first try to play just the Right Hand alone 3 times without error, then the Left Hand alone 3 times without error. If they are able to get that accomplished, then they should try playing Hands Together again.
Let me know if you have questions and happy practicing!