Your scenario is not a strict migration, as you've indicated, so we don't have any documentation to walk you through the process, but the following steps should help.
Make two copies/backups (e.g. db_B and db_C) of the SW database for your site A installation. For more information about creating database backups, see the SolarWinds Technical Reference, "Moving Your Orion NPM Database".
Complete the installation of your Site B and Site C installations, using databases db_B and db_C, as indicated in the SolarWinds Orion NPM Administrator Guide.
Note: You will want to delete duplicate objects.
Configure your EOC installation on Site A, as indicated in the SolarWinds Orion EOC Administrator Guide.