Using simpleSAML and setting up site 1 as a SAML identity provider should work for you:

https://drupal.org/project/simplesamlphp_auth

Jamie Holly
http://hollyit.net

On 12/4/2013 10:51 PM, Swatee Karpe wrote:

3rd site can shared user table and logged in with same credentials of 1.

4. So on.

As my requirement is not for sites who will come in same domain they have different domain so some modules have restriction on two different domain.

Like Bakery, SSO [ not for drupal 7], so on..

Do some have to know any related module?