Setting Up Cross Browser Support for Multiple Domains in .htaccess

I am running a number of api scripts off of a single server. A number of outside sites access these scripts from different domains.

When I use this command, it works fine.

Header add Access-Control-Allow-Origin "http://example1.com"

My online research told me that this was the way to set up the .htaccess file for multiple domains, but it isn't working.

SetEnvIf Origin "^http(s)?://(.+\.)?(example1\.com|example2\.com | example3\.org)$" origin_is=$0
Header always set Access-Control-Allow-Origin %{origin_is}e env=origin_is


Can you provide assistance as to how this .htaccess file needs to be set up to provide cross browswer support?

Thanks.