Cake Recipe found here
Adapted from Demolition Desserts, Elizabeth Falkner
Set a small amount aside for piping the train tracks, if you so desire. Gel food coloring is the best choice here, as it won't alter the consistency of your buttercream.
Once the cake is cooled, stick the layers in the freezer for at least a few hours (overnight is best, so they can be made a few days in advance).
For the number "3", take each layer of the cake and remove an 8cm circle from the middle. I made a paper circle template, put it on top of the cake, and cut around it. Remove about a 1/4 of one cake for the base of the three and about 1/3 of the remaining cake for the top. Position to make the number, trimming as required. It's best to put everything together on a cake board, as the final product is heavy. I cover my cake boards with foil because I like the way it looks.
Pipe your frosting in a wall around the bottom layer and fill with jam. Top with cake. Repeat for the other part of the 3. Using a light hand, go over the entire cake with frosting for your crumb coat. This doesn't have to be perfect; in fact, the point of this is to lock in the crumbs so it doesn't ruin your final layer of frosting. Just get everything thinly covered and pop it all in the fridge for at least an hour.
Note: the inside of the 3 was more difficult to frost than I expected. It's the inside of the cake, therefore it was crumbier and took more frosting to cover. I was glad I made a triple batch of buttercream.
Cover the cake with your final layer of buttercream and while it's still wet, stick on the Kit Kats for train tracks. If you're using M&M's, do those around the base now as well. Pipe the train rails carefully, and then set it all in the fridge to chill. I stored it there overnight because I didn't want to keep buttercream out that long.