This will likely come without effort, but I wanted to give this a solution since it seems the Windows 8 version has less activity compared to the Windows Phone version even though they basically play the same way.
In order to get 3 stars on a level you need to score a combination of 45 points (or more). The points are earned by either picking up coins (worth 2 points each) or by adding up points based on the zones the disks end up in at the end of the level. If you land a disk on a zone but it gets knocked out by the end it will not count.
I don't think there is a specific level that is better than others as most are pretty easy with a few retries. Level 1-3 is pretty simple since you can just throw all disks on the far left at high power and with some luck you'll get a few in the 15 point zone.
You will ultimately need to get 3 stars on all levels to unlock:
I don't feel like reinventing the wheel here so I'd recommend checking out the Challenges portion of the Walkthrough for the Windows Phone version of Shuffle Party here:
Shuffle Party (WP) Walkthrough Page 5 - Challenges & Shopping