Use a file like this:

Note: ID is to uniquely identify the record - it is like a social security number or national identification number for the record [your previous system may already have one] and is needed because you will be importing TWICE.

The first import will create the records without any relationships.

The second import will update the record to add the relationship.

ID is also used if you are importing prior to Go Live and want to do another import at Go Live, so you can UPDATE any record where emails or phone numbers change and ADD any record where new Companies or People are added.

Your first import will be like this:

Note that you are NOT importing the Parent (because it doesn't exist and unless you can sort your file so that all grandparents and parents are before all children Odoo won't find the Parent because it won't be created)

Your second import will be like this:

When you are done with both imports, you will have this: