I have three entities:
Airwaybills
Packages
Items
One Airwaybill can have many packages, each package can have many items
I want the user to go through creating Airwaybill record and then starts to add packages and finally items to the packages.
How is this setup is done on AppSheet?
You need to work with references between tables, sir.
First, understand that what you have described is not a many-to-many relationship. An example of many-to-many would be between a Doctors table and a Patients table. A single doctor will have many patients. But also a single patient may have many doctors that they see.
In your case, you have two levels of one-to many relationships. We refer to these as Parent/Child relationships. More precisely in your use case - GrandParent / Parent / Child
You do want to refer to the article noted by @diogolupcosta . Scroll down to the “Express ownership between tables” section. While it doesn’t name it, this section is describing how to setup an explicit or “tightly bound” Parent/Child relationship. Following this description will result in a properly established relationship where many features are created for you automatically by AppSheet. Be sure to ask questions if you need more help.
I hope this helps!!