Pitfalls of creating a new Site in AD

I’m new to this company which has 3 sites with a DC at each site. Active Directory Sites/Services has not been set up properly and users are authenticating across the WAN, even though the local DC is under utilized. I plan on creating 3 sites, with site links, and place the controllers in those sites. Mgmt wants to have a backout plan and what are the expected issues if any. For example if the DC goes down in Site A, will those users still be able to authenticate across the WAN to site B?
I’ve seen a few articles to perform this, but would like to see step by step instructions so I don’t miss something so if anyone has something I’d be very grateful…
Thanks