hi, does jms multi sites v3 already have single sign in capability or is the single sign in plug in /extension required to be purchased separately to achieve single sign in?

thanks

The SSI for domains is only required when you want to keep the users logged when they change of websites.
Normally, this additional extension is not required when you are using JMS 1.3 with users that are on either subdirectory or subdomains of a same domain.

So at the question, do you need to buy the "SSI for domain" in addition of JMS, the answer is Yes in case where you requirement is to remain logged between different domain name. Otherwise in most case, you don't need it.

Remark: Some hosting companies are using "suhosin" apache module to increase the security. In this case, the SSI for domain may also be required for the subdomains because the native joomla "cookie domain" can not be used to share the session information.