Add about ½ cup each color frosting to its own piping bag. You will have 3 bags.  

Snip the end off the piping bags. 

Prepare a fourth piping bag with the open star tip. 

Slide the three other piping bags into the fourth and twist the open ends to force the frosting down into the tip.

Pipe frosting over cupcake in a circular direction working from the inside of the cupcake towards the outer circle and back to the middle to create a peak.

Roll fondant flat and cut into triangles. 

Press one triangle into each cupcake at the peak of the frosting.

Decorate with sprinkles.