Asterisk Realtime newbie question

I have setup asterisksuccessfully. My problem is that, I have 3 context or I have 3 different server with different extensions, sip profiles. I wanted each server to connect with each other.Meaning to say the extensions can be dialed from one server to another. How am going to do that in asterisk realtime(with mysql database)? On a simplier setup I just have to add “include=>xxx,1,xxx” on the extensions.conf. Please help me do this configuration. Thanks

Best regards,