You can add new company as a parent company for all the 3 companies. (or)

You can add all the 3 companies as allowed companies for users of new company (or) 

You can create a new group and create a record rule to display all other companies SO and PO (but you have to make the default multi-company rule from global to any other group/groups.)

Hello Abdul Razak,

You can select multiple companies whose PO and So you want to show.

For Example :

I have 5 SO in my company(San Francisco). I want to show them in my company(Chicago). So i will select both the company to view San Francisco's SO and PO.

Hope it will help you.

Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari