This is a marble machine with a lift that lifts three (or more) marbles simultaneously, and releases them to three separate tracks. Two of those tracks are connected in series, so a marble will therefore alternate between those tracks. The third track loops back onto itself, so to have traffic in all three tracks, you need at least two marbles.
The machine has been designed for 8 mm balls as marbles, and the machine prints completely without supports if you have a well tuned printer. 8 mm steel balls will work fine, but I have also included a printed marble that can be used. The nice thing about printed marbles is that you can print several in different colors, which looks quite cool when running the machine. They also run a little slower than steel balls due to a little more friction against the tracks.
Total amount of plastic to print this will be around 300 grams.