So you have 3 different domains Contoso.com, Fabrikam.com and Redmond.com in different locations and want to merge or consolidate to single global domain? If answer yes - you will have setup a new AD Domain example - Newunifiedcorp.com domain and migrate users and objects from all the domains to this domain, however you will need network connectivity between all the domains and sites, read through the ADMT tool to consolidate and migrate the users and objects from the domain.
Hope this helps.
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