excuse me，Does the SSO service need to be built separately? What are the requirements?

Yes, you need to write the "fetch user" part from your website/framework

Skeleton project: https://github.com/evnix/codoforum-sso

Are you using a custom website ?

Thank you very much for your answer.

About 3 sites, plan to use Codoforum all.
Unified login between the three sites.

Hi,

Do the three sites use different logins or is there any central site for login ?

Or, did you mean 3 different instances of codoforum separately for the 3 websites ?

3 different instances of codoforum separately for the 3 websites,Yes!
The three sites are similar in content but belong to different categories, so three different domain names are used.
Similar to WordPress itself with multi-site features。
3 sites want to share the account system, can it be achieved?

Hi,

Yes, it is possible. If all your websites use the same authentication system, then it will be easier.

You need to follow the documentation here: https://codoforum.com/documentation/implementing-codoforum-sso

Does your website use any CMS or framework ?