API v3, how to add dropship instructions?

The instructions which will be sent to the supplier in dropship request email.

The text for the drop ship email comes from the packing list.

I see Packing List endpoints in API v2, but do not see how to set the list for an order.
Also, are Packing Lists available in v3?

I’m not sure what you mean by

Please clarify what is a Packing List and how to assign it to an order?

A packing list is in settings in the app.
Docs link.
The packing list displays what is in the order to be shipped, lists the items and quantity in the order.

You can adjust the custom text to display on the packing list.

Every order will use the template. For your case, you would need to adjust the template before sending the dropship request.