Have a look at this image http://www.cbucn.com/uploads/201409/cc8d1ceca032a49c509328a001a899d0.jpg
I'm going to create something similar.
Circles can be made very easily.
Color Gradient can be applied easily.
Shadows are left-hand play.
But, there is something different about the edges of those circles. They are kind of think or like give us a feel of thickness. I wonder how that is done with a flat shape?