Elias here with Shopify Support. Thanks for reaching out!
No problem at all! We've got a step-by-step guide, with a video tutorial, on how to set up drop-down menus in your online store. Simply create the 3 menu items for your columns and for the rest of your menu items. After that, click on the six dots to drag the menu items and you can move them to their respective columns. Dragging the menu items right will create the drop-down and you'll notice an indentation.
Here's an example screenshot from my test store:
I hope this helps! If you have any other questions on this topic, don't hesitate to let me know!