I plumbed my GS3 3 years ago and did not do it correctly which I regret daily. I was motivated to do it right when I saw my daughter-in-law's setup! I am reviving this old chain that discussed this topic originally. Most of us should have a screw-in hole at the back of the drain box and this post is for you. If you don't have threads in the drain box hole, read the original post I am reviving:

La Marzocco GS3 drain box connector

I had purchased a brass hose barb from home depot that screws into the back of the drain box, two 90-degree brass barb elbows and clear plastic tubing that was 5/8" ID. The whole thing is a mess and clogs easily because the plastic drain is not wide enough and the surface gunks up easily. This all started with the objective of saving money! The stock plumb-in kit costs $150 and includes the fitting for water inlet which I don't need. The brass barb piece alone at the expensive coffee places is $36. I was able to get the two parts needed at a fraction of the cost: the hose barb at Black Rabbit for $18 and 10 foot flexible drain on Amazon for $11. Links below:

3/4" ID Hose barb:

https://blackrabbitservice.com/products ... -for-drain

3/4" ID Flexible Drain:
PLASTIFLEX Smooth-BOR 90 3/4" X 10' Fill Hose https://a.co/d/81sorEB

The expensive stuff:

https://clivecoffee.com/products/linea-mini-plumb-kit

Hope you find this helpful. Use some teflon tape for the barb at the back of the drain box. The drain hose is super flexible so you will be able to bend it easily to its destination. In my case I route it to my shower drain above the p-trap. Good luck and do it right the first time!!