Single Sign-On Solution for Multiple Websites Using WordPress

I have three websites. If a client registers and logs in to one of these sites, can we store their usernames and passwords so that they are automatically logged in to the other two sites? I am using WordPress for this purpose. For the signup process, I plan to use Gravity Forms, but I am open to other suggestions. Is this functionality possible with the plugin? Additionally, all three of my sites are hosted on SiteGround. If any of the three sites are not built on WordPress, what would the process be? Thank you!