For the 2 cups of batter in the separate bowl, add in pistachio pudding mix and vanilla. Stir by hand until fully incorporated. Pour into a prepared 8-inch round cake pan and bake for 22-26 minutes.
My pistachio cake needed to bake for a bit longer than the cherry cake, but you can certainly bake them at the same time and just remove the cherry one first.
Prepare chocolate cake. (recipe linked to above) Bake as directed when the other cakes are done.
Allow all cakes to cool for about 5 minutes then invert onto a cooling rack.
Prepare chocolate, cherry and pistachio buttercream. (recipes linked to above) Place buttercream each into a pastry bag fitted with an open star tip.
Once chocolate cake is cooled, level the chocolate cake, the cherry cake and the pistachio cake to the same height.
To assemble cake, place chocolate cake on cake stand and cover in about 1/2 cup of chocolate buttercream. Place cherry cake on top of chocolate and cover in cherry buttercream. Place pistachio cake on top of cherry. (I did not do a crumb coat.)
Starting at the bottom and with chocolate buttercream, cover bottom layer with rosettes. (video and picture tutorial linked to above). Next do a row of cherry buttercream rosettes. Next a row of pistachio buttercream rosettes. The entire sides of the cake should be covered.
Finish covering the top with pistachio rosettes and chill until ready to serve.