website Single log in required for multiple sites

Description

I currently have two websites, they are as follows
https://knightstallion.com/https://shop.knightstallion.com/
i will soon be developing a third site which will be
book.knightstallion.com media.knighstallion.com
all os them will be using the same website theme/template which i have purchased from Themeforest
i am seeking someone who can create a single log in, for users, so that they can view all their purchases, rewards etc in one place, for convivence, as i believe this will be the best solution for users of my sites
i am seeking a professional, with experience, who will do the work and has the skill and knowledge to complete it
i will place a price as a holding price, the final cost will be negotiated with the right candidate