This can only be unlocked in career mode and more easily on Star difficulty.
Once you make the live shows, you'll see your placement by how high your percentage is of the overall votes after you sing. At each live show, you'll have a placement. If you finish first, your bar will be golden instead of red, which makes for easy checking.
If you skipped your placement, you can check all your placements at the TV in the hub area before singing.
After your third live show win, the achievement will pop once you are in the hub area.
Finally, winning the final show does NOT count towards the three live show wins. You'll need to have already unlocked the achievement before singing in the final.