Multi-site Live Preview not working in Craft 3 In the multi-site Live Preview in admin panel show error in the console.


  Failed to load http://www.example.local/test: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. Origin 'http://www.example.test' is therefore not allowed access. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.
Update

This has been implemented for the upcoming Craft 3.1 release.



Currently Live Preview will only work across sub-domains (i.e. site1.domain.com and site2.domain.com), not completely different domains (site1.com and site2.com) 
 Here's a guide on how you'd set that up: https://craftcms.com/guides/using-live-preview-across-multiple-subdomains

In order for it to work across completely different domains, https://github.com/craftcms/cms/issues/1521 will need to be implemented.