This is Dallas from the Shopify Social Care Team.
When it comes to using Oberlo there isn't a way to automatically bundle products to that your customers can purchase one and tow others will be added to Oberlo for free. The way that Oberlo works you'd have to have your customer order one product and then when you make the purchase on Oberlo yourself you can manually add the other two products to the order to be shipped to them.
If you were shipping out the products yourself you could do what's called product bundling. This would mean that when one product is ordered on Shopify the other two items could be added to the Order Confirmation for free. However, this, unfortunately, isn't compatible for usage with dropshipping companies.
If you wanted the items to be added to the Oberlo order you could upload the Y & Z products to your store and list them for free and then you could have a banner that says something like "If you order X don't forget to add Y & Z to your cart for free!" It's not automatic for your customers, but that is an alternative workaround.
I know this may not be the answer that you want to hear, but does that help to answer your question? If you have any other questions please let me know.