Hi,
We have facility in back-office to generate robot.txt file. It will be best practice as experienced people from prestashop team written code for SEO feature in back-office.
If you have 3 multi store websites makes sure you have unique content for each store. Duplicate content like title,keywords,meta description and content of the websites not good for SEO.
PrestaShop Tutorials Videos [How to do Tasks]
https://www.prestashop.com/forums/topic/907438-prestashop-tutorials-videos-how-to-do-tasks/