Is Multisite best solution for single user login to multiple domains?

I’ve read copious posts related to this issue, but remain unsure whether to risk merging 3 websites (individual WP installs for the www and 2 subdomains, as well as themes, plugins MySQL DBs etc.,) into a single Multisite network.

I am almost ready to launch my business, but realize that I overlooked the user registration issue between domains. It would be better when a user registers on any of the 3 websites, that their username and password (with profile, if possible) allows them to navigate freely between the sites and purchase items without having to create a new account on each domain.

Each domain represents a different product/service and is accessed via a similar menu bar on each site, so it makes sense that a logged-in user can simply click “buy now” on any website without having to log-in again!

If I merge the websites into one Multisite, will that resolve my issue, or is their another way (plugin or other) to achieve the desired outcome?

BTW, I am relatively new to online business (clearly!) and not an expert WP developer, but do have good knowledge of the product now.
Many Thanks ??