If the tops of the cakes are domed or uneven, carefully trim off the domed portion for a flat surface.
Place the vanilla cake on a cake board. Frost the top of the vanilla cake with a layer of vanilla buttercream, between 1//8-1/4 inch thick. Frost the sides of the cake with a thin coat of vanilla buttercream.
Place the strawberry cake on the vanilla cake. Frost the top of the strawberry cake with a layer of strawberry buttercream, between 1//8-1/4 inch thick. Frost the sides of the cake with a thin coat of strawberry buttercream.
Place the chocolate cake on the strawberry cake. Frost the top of the chocolate cake with a layer of chocolate buttercream, between 1//8-1/4 inch thick. Frost the sides of the cake with a thin coat of chocolate buttercream.
Take a bench scraper and drag it around the sides of the cake, to smooth the sides out and scrape off any excess frosting. Place the cake in the refrigerator for 30 minutes to chill and set the crumb coat.
For the final, "pretty" coat of buttercream, you can either use an icing spatula (which I did), or use separate piping bags for each color. Piping bags will give you cleaner, sharper lines between each color, but as you can see, if you're careful with an icing spatula, you can also keep it nice and neat.
Use your icing spatula to spread vanilla buttercream along the bottom edge, trying to keep it neat, filling in the bottom third of the cake all the way around. Repeat with the strawberry buttercream, and then chocolate, frosting the sides and top with chocolate. If you see any gaps, go ahead and dab on some more frosting to fill in the gaps.
Now, take your bench scraper again, hold it at about a 45-degree angle to the cake and lightly sweep it around the sides of the cake on a continuous motion – this is where it is extremely beneficial to decorate your cake on a turntable so that you can turn it with one hand while you smooth the buttercream with the other. Without a turntable, this step will be difficult for you. You are not trying to remove much frosting, so use a light hand – you are simply smoothing out any air bubbles and blending the colors together. Sweep around the cake several times, scraping the excess frosting off the bench scraper and into a bowl. Wipe the bench scraper clean with a paper towel each time around, and keep smoothing until you’re satisfied with the result. Smooth out the top. If you don't have perfectly smooth lines, don't worry. This technique takes some practice.