Connecting 3 sites

I want to connect a main office and 2 branch offices, all sites are running windows servers, from my research i have learned that connecting using VPN is the best option. has anybody come accross a step by step guide to inter connect three sites, such that there is one domain controller in every site and one domain span across 3 geographical region in case one domain controller (DC) goes down, the DC at the any other location takes over.

Please help